Extreme ultraviolet light source

ABSTRACT

The present invention provides a reliable, high-repetition rate, production line compatible high energy photon source. A very hot plasma containing an active material is produced in vacuum chamber. The active material is an atomic element having an emission line within a desired extreme ultraviolet (EUV) range. A pulse power source comprising a charging capacitor and a magnetic compression circuit comprising a pulse transformer, provides electrical pulses having sufficient energy and electrical potential sufficient to produce the EUV light at an intermediate focus at rates in excess of 5 Watts. In preferred embodiments designed by Applicants in-band, EUV light energy at the intermediate focus is 45 Watts extendable to 105.8 Watts.

This application is a continuation of U.S. Ser. No. 10/409,254, filedApr. 8, 2003, now U.S. Pat. No. 6,972,421, which is acontinuation-in-part of U.S. Ser. No. 10/384,967, filed Mar. 8, 2003,now U.S. Pat. No. 6,904,073, which is a continuation-in-part of Ser. No.10/189,824, filed Jul. 3, 2002, now U.S. Pat. No. 6,815,700, which is acontinuation-in-part of U.S. Ser. No. 10/120,655, filed Apr. 10, 2002,now U.S. Pat. No. 6,744,060, which is a continuation-in-part of U.S.Ser. No. 09/875,719, filed Jun. 6, 2001, now U.S. Pat. No. 6,586,757,which is a continuation-in-part of U.S. Ser. No. 09/875,721, filed Jun.6, 2001, now U.S. Pat. No. 6,566,668, which is a continuation-in-part ofU.S. Ser. No. 09/690,084, filed Oct. 16, 2000 now U.S. Pat. No.6,566,667; and U.S. Ser. No. 10/409,254 claims the benefit of U.S. Ser.No. 60/422,808, filed Oct. 31, 2002 and U.S. Ser. No. 60/419,805, filedOct. 18, 2002; all of which are incorporated by reference herein. Thisinvention relates to high-energy photon sources and in particular highlyreliable x-ray and high-energy ultraviolet sources.

BACKGROUND OF THE INVENTION

The semiconductor industry continues to develop lithographictechnologies, which can print ever-smaller integrated circuitdimensions. These systems must have high reliability, cost effectivethroughput, and reasonable process latitude. The integrated circuitfabrication industry has recently changed over from mercury G-line (436nm) and I-line (365 nm) exposure sources to 248 nm and 193 nm excimerlaser sources. This transition was precipitated by the need for higherlithographic resolution with minimum loss in depth-of-focus.

The demands of the integrated circuit industry will soon exceed theresolution capabilities of 193 nm exposure sources, thus creating a needfor a reliable exposure source at a wavelength significantly shorterthan 193 nm. An excimer line exists at 157 nm, but optical materialswith sufficient transmission at this wavelength and sufficiently highoptical quality are difficult to obtain. Therefore, all-reflectiveimaging systems may be required. An all reflective optical systemrequires a smaller numerical aperture (NA) than the transmissivesystems. The loss in resolution caused by the smaller NA can only bemade up by reducing the wavelength by a large factor. Thus, a lightsource in the range of 10 to 20 nm is required if the resolution ofoptical lithography is to be improved beyond that achieved with 193 nmor 157 nm. Optical components for light at wavelengths below 157 nm arevery limited. However, effective incidents reflectors are available andgood reflectors multi-layer at near normal angles of incidence can bemade for light in the wavelength range of between about 10 and 14 nm.(Light in this wavelength range is within a spectral range known asextreme ultraviolet light and some would light in this range, softx-rays.) For these reasons there is a need for a good reliable lightsource at wavelengths in this range such as of about 13.5 nm.

The present state of the art in high energy ultraviolet and x-raysources utilizes plasmas produced by bombarding various target materialswith laser beams, electrons or other particles. Solid targets have beenused, but the debris created by ablation of the solid target hasdetrimental effects on various components of a system intended forproduction line operation. A proposed solution to the debris problem isto use a frozen liquid or liquidfied or frozen gas target so that thedebris will not plate out onto the optical equipment. However, none ofthese systems have so far proven to be practical for production lineoperation.

It has been well known for many years that x-rays and high energyultraviolet radiation could be produced in a plasma pinch operation. Ina plasma pinch an electric current is passed through a plasma in one ofseveral possible configuration such that the magnetic field created bythe flowing electric current accelerates the electrons and ions in theplasma into a tiny volume with sufficient energy to cause substantialstripping of outer electrons from the ions and a consequent productionof x-rays and high energy ultraviolet radiation. Various prior arttechniques for generation of high energy radiation from focusing orpinching plasmas are described in the background section of U.S. Pat.No. 6,452,199.

Typical prior art plasma focus devices can generate large amounts ofradiation suitable for proximity x-ray lithography, but are limited inrepetition rate due to large per pulse electrical energy requirements,and short lived internal components. The stored electrical energyrequirements for these systems range from 1 kJ to 100 kJ. The repetitionrates typically did not exceed a few pulses per second.

What is needed are production line reliable, systems for producingcollecting and directing high energy ultraviolet x-radiation withindesired wavelength ranges which can operate reliably at high repetitionrates and avoid prior art problems associated with debris formation.

SUMMARY OF THE INVENTION

The present invention provides a reliable, high-repetition rate,production line compatible high energy photon source. A very hot plasmacontaining an active material is produced in vacuum chamber. The activematerial is an atomic element having an emission line within a desiredextreme ultraviolet (EUV) wavelength range. A pulse power source,comprising a charging capacitor and a magnetic compression circuitcomprising a pulse transformer, provides electrical pulses havingsufficient energy and electrical potential sufficient to produce the EUVlight at an intermediate focus at rates in excess of 5 Watts on acontinuous basis and in excess of 20 Watts on a burst basis. Inpreferred embodiments designed by Applicants in-band, EUV light energyat the intermediate focus is 45 Watts extendable to 105.8 Watts.

In preferred embodiments the high energy photon source is a dense plasmafocus device with co-axial electrodes. The electrodes are configuredco-axially. The central electrode is preferably hollow and an active gasis introduced out of the hollow electrode. This permits an optimizationof the spectral line source and a separate optimization of a buffer gas.In preferred embodiments the central electrode is pulsed with a highnegative electrical pulse so that the central electrode functions as ahollow cathode. Preferred embodiments present optimization ofcapacitance values, anode length and shape and preferred active gasdelivery systems are disclosed. Special techniques are described forcooling the central electrode. In one example, water is circulatedthrough the walls of the hollow electrode. In another example, a heatpipe cooling system is described for cooling the central electrode.

An external reflection radiation collector-director collects radiationproduced in the plasma pinch and directs the radiation in a desireddirection. Good choices for the reflector material are molybdenum,palladium, ruthenium, rhodium, gold or tungsten. In preferredembodiments the active material may be xenon, lithium vapor, tin vaporand the buffer gas is helium and the radiation-collector is made of orcoated with a material possessing high grazing incidence reflectivity.Other potential active materials are described.

In preferred embodiments the buffer gas is helium or argon. Lithiumvapor may be produced by vaporization of solid or liquid lithium locatedin a hole along the axis of the central electrode of a coaxial electrodeconfiguration. Lithium may also be provided in solutions since alkalimetals dissolve in amines. A lithium solution in ammonia (NH₃) is a goodcandidate. Lithium may also be provided by a sputtering process in whichpre-ionization discharges serves the double purpose of providing lithiumvapor and also pre-ionization. In preferred embodiments, debris iscollected on a conical nested debris collector having surfaces alignedwith light rays extending out from the pinch site and directed towardthe radiation collector-director. The reflection radiationcollector-director and the conical nested debris collector could befabricated together as one part or they could be separate parts alignedwith each other and the pinch site.

This prototype devices actually built and test by Applicants convertelectrical pulses (either positive or negative) of about 10 J of storedelectrical energy per pulse into approximately 50 mJ of in-band 13.5 nmradiation emitted into 2π steradians. Thus, these tests havedemonstrated a conversion efficiency of about 0.5%, Applicants estimatethat they can collect about 20 percent of the 50 mJ 13.5 nm radiation sothat this demonstrated collected energy per pulse will be in about of 10mJ. Applicants have demonstrated 1000 Hz continuous operation and 4000Hz short burst operation. Thus, 10 Watt continuous and 40 Watt burstoutputs have been demonstrated. Using collection techniques designed byApplicants about half of this energy can be delivered to an intermediatefocus distant from the plasma source. Thus providing at least 5 Watts ofin band EUV light at the intermediate focus on a continuous basis and atleast 20 Watts on a burst basis. Applicants have also shown that thetechniques described herein can be applied to provide outputs in therange of 60 Watts at repetition rates of 5,000 Hz or greater. At 2000Hz, the measured pulse-to-pulse energy stability, (standard deviation)was about 9.4% and no drop out pulses were observed. The electricalcircuit and operation of this prototype DPF device is presented alongwith a description of several preferred modifications intended toimprove stability, efficiency and performance.

In other embodiments the plasma may be produced in other plasma pinchdevices such as a conventional z pinch device, a hollow cathode z-pinchor a capillary discharge or the plasma may be produced with a pulsed gasdischarge laser beam. The pulse power or each of these sources isproduced with a pulse power system as described herein and in each theEUV light preferably is produced collected and is preferably deliveredto an intermediate focus using one or more of the techniques describedherein.

The present invention provides a practical implementation of EUVlithography in a reliable, high brightness EUV light source withemission characteristics well matched to the reflection band of theMo/Si or Mo/Be mirror systems. Tests by Applicants have demonstrated animproved electrode configuration in which the central electrodeconfiguration in which the central electrode is hollow and configured asa cathode. For this configuration the hollow cathode produces its ownpre-ionization so special pre-ionization is not needed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electrical drawing of a pulse power system useful as apower source for EUV and soft x-ray sources.

FIG. 1A shows structure elements of a pulse transformer.

FIGS. 1B and 1C show test data.

FIG. 1D shows a reverse polarity pulse power source.

FIG. 2A shows electrical features of a dense plasma focus EUV device.

FIGS. 2A(1) and 2A(2) are cross-section drawings of a plasma pinchprototype EUV device.

FIG. 2A(3) shows the prototype with vacuum chamber.

FIG. 2A(4) shows flow cooled equipment.

FIG. 2A(5) shows effects of flow on output.

FIGS. 2A(6)-(20) show performance data.

FIG. 2A(21) shows a special DPF configuration.

FIG. 2B shows features of a conventional Z-pinch device.

FIG. 2C shows features of a hollow cathode Z-pinch device.

FIG. 2D shows features of a capillary discharge device.

FIGS. 3A and 3B show xenon spectra.

FIGS. 4, 4A and 4B show features of a laser produced plasma systems.

FIG. 4C shows a hybrid EUV system.

FIG. 5A-C shows methods of making a debris collector.

FIGS. 6A and 6B show features of a second debris collector.

FIGS. 7A, 7B, and 7C show features of a third debris collector.

FIG. 8 illustrates the reflectivity of a multilayer mirror in thespectrum for lithium.

FIG. 8A is a prospective drawing of a hyprobolic collector.

FIG. 8B shows a portion of the EUV beam produced by an ellipsoidalcollector.

FIG. 8C shows a portion of the EUV beam produced by a hyproboliccollector.

FIGS. 9, 9A, 9B, and 9C show combinations of radiation collectors and adebris collector.

FIG. 10 shows a xenon spectra a multi-layer mirror spectra.

FIG. 11A is a chart showing reflectivity of various materials for 13.5mn ultraviolet radiation.

FIGS. 11B, 11C, 11D, and 11E show collector designs.

FIG. 12 is a drawing showing a technique for introducing source gas andworking gas.

FIG. 13 is a time chart showing the anode voltage and EUV intensity.

FIGS. 14A, 14B, 14C and 14D show the effect of various central electrodedesigns on the plasma pinch.

FIG. 15 is a drawing showing a technique for using RF energy to operatelithium vapor source gas.

FIG. 16 shows a heat pipe cooling technique for the anode in a preferredDPF device.

FIG. 17 shows gas control techniques.

FIGS. 18A, 18B, 18C, and 18D show techniques for controlling active gasand buffer gas in the vacuum vessel of preferred embodiments.

FIG. 19 shows a tandem ellipsoidal mirror arrangement.

FIGS. 19A, 19B, and 19C show the shape of the EUV profile at justdownstream of two focuses.

FIGS. 20, 20A, 21 and 22 show a technique for water-cooling of theelectrodes.

FIGS. 23, 24, 25, 26A and 26B show techniques for reducing electrodeerosion.

FIG. 25 shows a technique for reducing electrode erosion.

FIGS. 27A and 27B show a maintenance technique.

FIGS. 28A and 28B show the use of magnets to control the pinch.

FIGS. 29A, 29B, and 29C show pulse shapes.

FIG. 30 shows a pulse shape.

FIG. 31 shows a preionization technique.

FIG. 32 shows the effects of preionization turning.

FIG. 33 shows advantages of dense plasma focus.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Hot Plamas

To produce light in the spectral range of 13-14 nm from plasma requiresa very hot plasma corresponding to temperatures of in the range ofseveral thousand degrees Celsius. Plasmas at these temperatures can becreated by focusing a very high power (very short pulse) laser beam or ahigh energy electron beam on the surface of a metal target. It is alsopossible to produce very hot plasma in a gas with electric dischargesusing any of several special discharge techniques which focus or pinchthe plasma.

These techniques included (1) a dense plasma focus technique (2) aregular Z-pinch technique, (3) a hollow cathode Z-pinch and (4) acapillary discharge technique. All of these techniques are discussed ingreater detail below. For use as a lithography light source forintegrated circuit fabrication the light source and the power supply forit should be capable of continuous, reliable, round-the-clock operationfor many billions of pulses. This is because the lithography machinesand the associated fabrication lines are extremely expensive and anyunscheduled down time could represent losses of hundreds of thousands ofdollars per hour.

Atomic Sources for 12-14 nm EUV Spectral Lines

As stated in the background section of this specification good mirrorsare available providing reflectances in the range of about 70% or higherin the wavelength the range of between about 10 and 14 nm. These mirrorstypically provide reflectances at these high values only over a morenarrow spectral range within the 12 to 14 nm range. For example, themirror depicted in FIG. 11A provides reflectances of about 70% over thespectral range of about 13.2 to 13.8 nm. This mirror can be described ashaving a reflectance of about 0.7 at 13.5 nm with a FWHM bandwidth of0.5 nm. These mirrors can be effectively utilized for lithographymachines for future integrated circuit lithography. Plasma producingdevices described below, including those shown in FIGS. 2A through 2Dproduce spot plasmas with extremely high temperatures in the range ofseveral thousand degrees Celsius but the spectrum of light emitted isspread over a very wide range. To produce light within the desired rangeof about 13-14 nm, hot spot plasma should include an atomic targetmaterial with spectral lines in the 13-14 nm range. Several potentialtarget materials are known including xenon, lithium and tin. The bestchoice of target materials involve trade-offs relating to spectraavailable, efficiency of conversion of plasma energy to energy in thedesired spectrum, difficulty of injecting the target into the plasmaregion, debris problems. Some preferred target and techniques fordealing with these issues are discussed below. (The reader shouldunderstand that all elements produce spectral lines at high temperatureand that these lines are well documented so that if light at otherwavelengths is desired, it is a rather straight forward matter to searchthe literature for a suitable target material which when heated in aplasma will produce a good line at the wavelength of interest).

Xenon

Xenon is a preferred atomic target. It is a nobel gas therefore, it doesnot present a debris problem. It has relatively good spectral lineswithin the 13 to 14 nm range as shown in FIGS. 3A and 3B. FIG. 11A showsEUV Xe spectra measured by Applicants. FIG. 3A shows a measured singlepulse spectrum. FIG. 3B shows a calculated theoretical Xe spectron. Itcan can be added as a constituent part of the buffer gas in thedischarge chamber, or it an be injected close to the discharge or pinchregion so that its concentration is greater there. It can also be cooledto below its boiling point and injected into the discharge or pinchregion as a liquid or a solid so that its atomic concentration isgreatly increased in the plasma. Certain xenon compounds (such as xenonoxifluoride) might also make good target materials.

Lithium

Lithium is also well known as a potential target material. It is a solidat standard temperatures and it does pose a debris problem. Also,special techniques must be devised when adding atomic lithium to thedischarge or pinch region. Some of those techniques are described in theparent patent applications and in prior art sources and other techniquesare described below. Lithium can be injected into the chamber as asolid, liquid or a vapor.

Tin

Tin is also a preferred target material since it has some intensespectral lines in the desired range. However, like lithium it is a solidat standard temperatures and does pose a debris problem since it couldpotentially plate out on optical surfaces.

Pulse Power System Electrical Circuit Need For Long Life Reliable PulsePower

Several prior art pulse power supply systems are known for supplyingshort electrical high voltage pulses to create the discharges in thesedevices. However, none of these prior art power supplies provides thereliability and control features needed for high repetition rate, highpower long-life and reliability needed for integrated circuitlithographic production. Applicants have, however, built and tested apulse power system relying in part on technology developed by Applicantsfor their excimer laser light sources. These excimer lasers producing248 nm and 193 nm light, are currently extensively used as light sourcesfor integrated circuit fabrication. A long life reliable pulse powersystem for EUV devices built and tested by Applicants as part of afourth generation plasma focus device is described in the followingsections.

A description of the electrical circuit diagram of this preferred pulsepower system with reference to FIG. 1 and occasionally to FIGS. 1A, 2Aand B is set forth below.

A conventional approximately 700 V dc power supply 400 is used toconvert AC electrical power from utility 208 Volt, 3 phase power intoapproximately 700 V dc 50 amp power. This power supply 400 providespower for resonant charger unit 402. Power supply unit 400 charges up alarge 1550 μF capacitor bank, C-1. Upon command from an external triggersignal, the resonant charger initiates a charging cycle by closing thecommand-charging switch, S1. Once the switch closes, a resonant circuitis formed from the C-1 capacitor, a charging inductor L1, and a C0capacitor bank which forms a part of solid pulse power system (SSPPS)404. Current therefore begins to discharge from C-1 through the L1inductor and into C0, charging up that capacitance. Because the C-1capacitance is much, much larger than the C0 capacitance, the voltage onC0 can achieve approximately 2 times the initial voltage of that on C-1during this resonant charging process. The charging current pulseassumes a half-sinusoidal shape and the voltage on C0 resembles a “oneminus cosine” waveform.

In order to control the end voltage on C0, several actions may takeplace. First, the command-charging switch S1 can be opened up at anytime during the normal charging cycle. In this case, current ceases toflow from C-1 but the current that has already been built up in thecharging inductor continues to flow into C0 through the free-wheelingdiode D3. This has the effect of stopping any further energy from C-1from transferring to C0. Only that energy left in the charging inductorL1 (which can be substantial) continues to transfer to C0 and charge itto a higher voltage.

In addition, the de-qing switch S2 across the charging inductor can beclosed, effectively short-circuiting the charging inductor and “de-qing”the resonant circuit. This essentially removes the inductor from theresonant circuit and prevents any further current in the inductor fromcontinuing to charge up C0. Current in the inductor is then shunted awayfrom the load and trapped in the loop made up of charging inductor L1,the de-qing switch S2, and the de-qing diode D4. Diode D4 is included inthe circuit since the IGBT has a reverse anti-parallel diode included inthe device that would normally conduct reverse current. As a result,diode D4 blocks this reverse current which might otherwise bypass thecharging inductor during the charging cycle. Finally, a “bleed down” orshunt switch and series resistor (both not shown in this preferredembodiment) can be used to discharge energy from C0 once the chargingcycle is completely finished in order to achieve very fine regulation ofthe voltage on C0.

The DC power supply is a 208 V, 90 A, AC input, 800 V, 50 A DC outputregulated voltage power supply provided by vendors such as UniversalVoltronics, Lambda/EMI, Kaiser Systems, Sorensen, etc. A secondembodiment can use multiple, lower power, power supplies connected inseries and/or parallel combinations in order to provide the totalvoltage, current, and average power requirements for the system. The C-1capacitor in the resonant charger 402 is comprised of two 450 V DC, 3100μF, electrolytic capacitors connected together in series. The resultingcapacitance is 1550 μF rated at 900 V, providing sufficient margin overthe typical 700-800 V operating range. These capacitors can be obtainedfrom vendors such as Sprague, Mallory, Aerovox, etc. The commandcharging switch S1 and output series switch S3 in the cycle. In thiscase, current ceases to flow from C-1 but the current that has alreadybeen built up in the charging inductor continues to flow into C0 throughthe free-wheeling diode D3. This has the effect of stopping any furtherenergy from C-1 from transferring to C0. Only that energy left in thecharging inductor L1 (which can be substantial) continues to transfer toC0 and charge it to a higher voltage.

In addition, the de-qing switch S2 across the charging inductor can beclosed, effectively short-circuiting the charging inductor and “de-qing”the resonant circuit. This essentially removes the inductor from theresonant circuit and prevents any further current in the inductor fromcontinuing to charge up C0. Current in the inductor is then shunted awayfrom the load and trapped in the loop made up of charging inductor L1,the de-qing switch S2, and the de-qing diode D4. Diode D4 is included inthe circuit since the IGBT has a reverse anti-parallel diode included inthe device that would normally conduct reverse current. As a result,diode D4 blocks this reverse current which might otherwise bypass thecharging inductor during the charging cycle. Finally, a “bleed down” orshunt switch and series resistor (both not shown in this preferredembodiment) can be used to discharge energy from C0 once the chargingcycle is completely finished in order to achieve very fine regulation ofthe voltage on C0.

The DC power supply is a 208 V, 90 A, AC input, 800 V, 50 A DC outputregulated voltage power supply provided by vendors such as UniversalVoltronics, Lambda/EMI, Kaiser Systems, Sorensen, etc. A secondembodiment can use multiple, lower power, power supplies connected inseries and/or parallel combinations in order to provide the totalvoltage, current, and average power requirements for the system. The C-1capacitor in the resonant charger 402 is comprised of two 450 V DC, 3100μF, electrolytic capacitors connected together in series. The resultingcapacitance is 1550 μF rated at 900 V, providing sufficient margin overthe typical 700-800 V operating range. These capacitors can be obtainedfrom vendors such as Sprague, Mallory, Aerovox, etc. The commandcharging switch S1 and output series switch S3 in the embodiment are1200 V, 300 A IGBT switches. The actual part number of the switches isCM300HA-24H from Powerex. The de-qing switch S2 is a 1700 V, 400 A IGBTswitch, also from Powerex, part number CM400HA-34H. The charginginductor L1 is a custom made inductor made with 2 sets of parallelwindings (20 turns each) of Litz wire made on a toroidal, 50-50% NiFetape wound core with two ⅛″ air gaps and a resulting inductance ofapproximately 140 μH. National Arnold provides the specific core. Otherembodiments can utilize different magnetic materials for the coreincluding Molypermaloy, Metglas, etc. The series, de-qing, andfreewheeling diodes are all 1400 V, 300 A diodes from Powerex, partnumber R6221430PS.

Once the resonant charger 402 charges up C0, a trigger is generated by acontrol unit (not shown) in the resonant charger that triggers the IGBTswitches S4 to close. Although only one is shown in the schematicdiagram (for clarity), S4 consists of eight parallel IGBT's which areused to discharge C0 into C1. Current from the C0 capacitors thendischarges through the IGBT's and into a first magnetic switch LS1.Sufficient volt-seconds are provided in the design of this magneticswitch to allow all of the 8 parallel IGBT's to fully turn on (i.e.close) prior to substantial current building up in the dischargecircuit. After closure the main current pulse is generated and used totransfer the energy from C0 into C1. The transfer time from C0 to C1 istypically on the order of 5 μs with the saturated inductance of LS1being approximately 230 nH. As the voltage on C1 builds up to the fulldesired voltage, the volt-seconds on a second magnetic switch LS2 runout and that switch saturates, transferring the energy on C1 into 1:4pulse transformer 406 which is described in more detail below. Thetransformer basically consists of three one turn primary “windings”connected in parallel and a single secondary “winding”. The secondaryconductor is tied to the high voltage terminal of the primaries with theresult that the step-up ratio becomes 1:4 instead of 1:3 in anauto-transformer configuration. The secondary “winding” is then tied toC2 capacitor bank that is then charged up by the transfer of energy fromC1 (through the pulse transformer). The transfer time from C1 to C2 isapproximately 500 ns with the saturated inductance of LS2 beingapproximately 2.3 nH. As the voltage builds up on C2, the volt-secondproduct of the third magnetic switch LS3 is achieved and it alsosaturates, transferring the voltage on C2 to anode 8 a as shown on FIGS.14A and 14B. The saturated inductance of LS3 is approximately 1.5 nH.

Bias circuitry shown in the FIG. 1 at 408 is also used to properly biasthe three magnetic switches. Current from the bias power supply V1,passes through magnetic switch LS3. It then splits and a portion of thecurrent passes through bias inductor L5 and back to the bias powersupply V1. The remainder of the current passes through the pulsetransformer secondary winding and then through magnetic switches LS2 andLS1 and bias inductor L3 back to the bias power supply V1. Bias inductorL2 provides a path back to the power supply from current through thepulse transformer primary to ground. Bias inductors L3 and L5 alsoprovide voltage isolation during the pulse in the SSPPS since the biaspower supply V1 operates close to ground potential (as opposed to thepotentials generated in the SSPPS where the bias connections are made).

The C0, C1 and C2 capacitances are made up of a number of parallel,polypropylene film capacitors mounted on a printed circuit board withthick (6-10 oz.) copper plating. The printed circuit boards are wedgeshaped such that 4 boards make up a cylindrical capacitor deck whichfeeds a cylindrical bus for both the high voltage and groundconnections. In such a way, a low inductance connection is formed whichis important to both the pulse compression and to the stability of theplasma pinch in the DPF itself. The total capacitance for C0 and C1 are21.6 μF each while the total capacitance for C2 is 1.33 μF The C0 and C1capacitors are 0.1 μF, 1600 V capacitors obtained from vendors such asWima in Germany or Vishay Roederstein in North Carolina. The C2capacitance is made up of three sections of capacitors stacked in seriesto achieve the overall voltage rating since the voltage on the secondaryof the pulse transformer is about 5 kV. The C2 capacitors are 0.01 μF,2000 V dc components, again from Wima or Vishay Roederstein. The SSPPSswitches are 1400 V, 1000 A IGBT switches. The actual part number isCM1000HA-28H from Powerex. As noted earlier, 8 parallel IGBT switchesare used to discharge C0 into C1. The SSPPS series diodes are all 1400V, 300 A diodes from Powerex, part number R6221430. Two diodes are usedfor each IGBT switch, resulting in a total of sixteen parallel devices.

Magnetic switch LS1 is a custom made inductor made with 16 sets ofparallel windings (6 turns each) of Litz wire made on a toroidal,ferrite core. The specific core is provided by Ceramic Magnetics of NewJersey and is made of CN-20 ferrite material. The toroid is 0.5″ thickwith an I.D. of 5.0″ and an O.D. of 8.0″. Magnetic switch LS2 is asingle turn, toroidal inductor. The magnetic core is tape wound on a8.875″ O.D. mandrel using 2″ wide, 0.7 mil thick, 2605-S3A Metglas fromHoneywell with 0.1 mil thick Mylar wound in between layers to an outsidediameter 10.94″. Magnetic switch LS3 is also a single turn, toroidalinductor. The magnetic core is tape wound on a 9.5″ O.D. mandrel using1″ wide, 0.7 mil thick, 2605-S3A Metglas from Honeywell with 0.1 milthick Mylar wound in between layers to an outside diameter of 10.94″.

The pulse transformer is shown at 406, also shown in FIG. 1A has threetransformer core. Each of the three transformer cores is tape wound on a12.8 inch O.D. mandrel 422 using 1″ wide, 0.7 mil thick, 2605-S3AMetglas

from Honeywell with 0.1 mil thick Mylar wound in between layers to anoutside diameter of 14.65″. Each of the three cores 418 are ring shaped,12.8 inch I.D. and about 14 inch O.D. having heights of 1 inch. FIG. 1Ais an axial cross section sketch showing the physical arrangement of thethree cores and the primary and secondary “windings”. Each of theprimary windings actually are formed from two circular rings 420A and420B bolted to mandrel 422 and rod-like spacers 424. The secondary“winding” is comprised of 48 circularly spaced bolts 426. Thetransformer operates on a principal similar to that of a linearaccelerator, as described in U.S. Pat. No. 5,142,166. A high voltagecurrent pulse in the three primary “windings” induce a voltage rise inthe secondary “winding” approximately equal to the primary voltage. Theresult is a voltage generated in the secondary winding (i.e., rods 426)equal to three times the primary voltage pulse. But since the lowvoltage side of the secondary winding is tied to the primary windings afour-fold transformation is provided in this “auto-transformer”configuration.

Bias inductors L3 and L4 are both toroidal inductors wound on aMolypermalloy magnetic core. The specific core dimensions are a heightof 0.8″, an I.D. of 3.094″, and an O.D. of 5.218″. The part number ofthe core is a-430026-2 from Group Arnold. Inductor l3 has 90 turns of 12AWG wire wound on the toroid for an inductance of ˜7.3 mH while L4 has140 turns of 12 AWG wire wound on it for an inductance of ˜18 mH. Biasinductor L6 is merely 16 turns of 12 AWG wire wound in a 6″ diameter.Bias inductor L4 is 30 turns of 12 AWG wire in a 6″ diameter. Biasinductor L2 is 8 turns of 12 AWG wire in a 6″ diameter. Resistor R1 isan array of twenty parallel resistors, each of which is 27 ohm, 2Wcarbon composition resistor.

Polarity

In a preferred embodiment of the present invention, the electricalcircuit as shown in FIG. 1 provides positive high voltage pulses to thecenter electrode 8A as shown in FIG. 2, FIG. 2B1, and FIG. 2B2. Thedirection of current flow of each portion of each initial pulse is shownby arrows 409A, 409B and 409C respectively through the primary andsecondary sides of the transformer 406 and between the electrodes. (Thereader should note the direction of electron flow is opposite thedirection of current flow). The reader should note also that during thelatter portion of each pulse the current actually reverses as indicatedby the trace shown at 409D in FIG. 1B so that the voltage on C2 rises toabout +4 kV then rises to about zero.

Reverse Polarity

In prior art dense plasma focus devices, the central electrode istypically configured as an anode with the surrounding electrodeconfigured as cathode. Thus, the polarity of the electrodes of theembodiment shown in FIG. 2B is consistent with this prior art technique.It is known in the prior art to reverse the polarity of the electrodes;however, the results have typically been a substantial reduction inperformance. (For example, see G. Decker, et al., “Experiments Solvingthe Polarity Riddle of the Plasma Focus,” Physics Letters, Vol. 89A,Number 8, 7 Jun. 1982).

Applicants have in a preferred embodiment of the present inventiondemonstrated excellent performance by reversing the electrode polarityof a dense plasma focus device. To do this Applicants modified thecircuit shown in FIG. 1 to provide a circuit as shown in FIG. 1D. Thebasic design of the FIG. 1 circuit made this task relatively easy. Theconnections on DC power supply 400 were switched, switches S1, S2, S3and S4 were reversed and diodes D1, D2, D3 and D4 were reversed. Alsothe polarity of bias power supply V1 was reversed. As a result theinitial current flow for each pulse was in the directions shown at 409A,409B, and 409C in FIG. 1D. Thus, the central electrode 8A as shown inthe figures including FIG. 2B2 is initially charged negative and theinitial current flow in this embodiment is from ground electrodes 8B tocentral electrode 8A. The electron flow is in the opposite direction;i.e., from central electrode 8A to surrounding electrode 8B. Anothertechnique for reversing polarity is to modify the pulse transformerdesign to eliminate the “onto” aspect of the transformer. That is toconnect the low voltage side to ground instead of the primary highvoltage. If this is done polarity can be reversed by merely changing thesecondary leads of the pulse transformer. This of course would mean inthis case there would be only a factor of 3 increase in voltage ratherthan 4. But to compensate another primary section could be added.

Applicants' experiments have demonstrated some surprising improvementsresulting from this change in polarity. An important improvement is thatpre-ionization requirements are greatly reduced and may be completelyeliminated. Applicants believe this improved performance results from ahollow-cathode type effect resulting from the hollow portion at the topof electrode 8A as shown in FIG. 2A. According to Applicantsmeasurements under various conditions, the quality of pinches is betterthan pinches produced with the positive central electrode polarity.Applicants estimate increases in EUV output could be up to about afactor of two.

Energy Recovery

In order to improve the overall efficiency this fourth generation denseplasma focus device provides for energy recovery on a pulse-to-pulsebasis of electrical pulse energy reflected from the discharge portion ofthe circuit. Energy recovery is achieved as explained below by referenceto FIG. 1.

After the discharge C2 is driven negative. When this occurs, LS2 isalready saturated for current flow from C1 to C2. Thus, instead ofhaving energy ringing in the device (which tends to cause electrodeerosion) the saturated state of LS2 causes the reverse charge on C2 tobe transferred resonantly back into C1. This transfer is accomplished bythe continued forward flow of current through LS2. After the transfer ofcharge from C2 to C1, C1 then has a negative potential as compared to C0(which at this time is at approximately ground potential) and (as wasthe case with LS2) LS1 continues to be forward conducting due to thelarge current flow during the pulse which has just occurred. As aconsequence, current flows from C0 to C1 bringing the potential of C1 upto about ground and producing a negative potential on C0.

The reader should note that this reverse energy transfers back to C0 ispossible only if all the saturable inductors (LS1, LS2 and LS3) remainforward conducting until all or substantially all the energy isrecovered on C0. After the waste energy is propagated back into C0, C0is negative with respect to its initial stored charge. At this pointswitch S4 is opened by the pulse power control. Inverting circuitcomprising inductor L1 and solid state diode D3 coupled to ground causesa reversal of the polarity of C0 as the result of resonant free wheeling(i.e., a half cycle of ringing of the L1-C0 circuit as clamped againstreversal of the current in inductor L1 by diode D3 with the net resultthat the energy is recovered by the partial recharging of C0. Therefore,the energy which otherwise would have contributed to the erosion of theelectrodes is recovered reducing the charging requirements for thefollowing pulse.

Importance of Output Switch

As shown in FIGS. 1 and 1D, the pulse power system described in thisinvention possesses an output switch that performs several functions.This switch, LS3 in the figure, is a saturable inductor which we referto as a magnetic switch. As explained above, it is biased by biascircuitry 408 so as to initially hold off current flow at the beginningof each pulse until the inductor saturates at which time current flowsfor about 100 nanoseconds after which the bias current re-biases theswitch prior to the start of the next pulse which at 5 kHz (for example)comes about 200 micro seconds later.

This switch is very important for allowing proper operation of thesource at high repetition rates. Although some EUV sources have beendeveloped without such a switch, their operation at high rep-rates canbe erratic in output energy. In these cases, no switch exists betweenthe energy storage capacitor and the EUV source load. The issue is thatthe source load may not fully recover in the short time between the lastpulse and the time when voltage is applied to the energy storagecapacitor in preparation for the next pulse. At rep-rates of 5 kHz, only200 μs exists between output pulses. With many of the other sourcedesigns, a significant fraction of this inter-pulse period would berequired for charging of the energy storage capacitor. Thus, evenshorter time may exist between the last pulse generation and the initialvoltage application across the capacitor (and also the load since nooutput switch exists to isolate the two). Problems then exist when thistime becomes too short for the plasma from the last pulse to cool downand recover (hold off voltage application in anticipation of the nextpulse). As a result, the source may breakdown again prematurely atlower-than-normal voltages when the recovery is not sufficient. Becausethe breakdown process is statistical in nature, there can also be widevariation in the breakdown voltages, leading to large variations insource output EUV energy levels. This causes significant problems in thelithography application since energy stability and dose control are veryimportant parameters for process control.

The advantage of the output switch, LS3 in the invention describedherein, is that it can perform several functions which help to eliminatethis issue of premature load breakdown. In the normal pulse generation,the LS3 switch acts as a magnetic switch and a diode to prevent currentreversal through the load. As a result, any energy not absorbed by theload is reflected back to the initial storage capacitor, C0 where theenergy is recovered and stored for use with the next pulse (as describedearlier in the section on Energy Recovery). In this manner, energy isquickly removed from the load after the main pulse generation and istherefore not allowed to continually oscillate until it is finally andcompletely dissipated in the load plasma. This helps to reduce theenergy deposited into the load plasma and allows it to begin therecovery process as soon as possible after the main pulse generation andEUV output. In addition, the LS3 output switch provides isolationbetween the last energy storage capacitor and the source load, allowingthe source additional time to recover prior to the next pulse beinggenerated. This switch allows the last energy storage capacitor C2,which we refer to as the discharge capacitor, to begin charging as soonas the LS3 switch is reverse biased after the energy recovery process iscompleted. The design of the bias circuit (including bias inductor L4and bias power supply V1) can be developed to allow LS3 recovery insufficient time for charging of C2 in the next pulse generation sequenceat rep-rates of at least 5 kHz. The LS3 switch is therefore initiallyreverse biased (not conducting in the forward direction—towards theload) up until the time when it saturates (as the voltage on C2 reachesits maximum value). The switch then allows energy transfer from C2 intothe load and remains forward conducting until the energy recovery cycleis completed and reflected energy is recovered all the way back onto C0.After this period of time, energy from the bias circuit is applied tothe main pulse compression circuit and completes the cycle by reversebiasing the LS3 switch again. Once this is accomplished, the charging ofC2 can take place again without the potential issue of the load breakingdown prematurely (since the LS3 switch can now isolate the voltage on C2from the load).

As rep-rates for EUV sources may eventually have to extend all the wayto 10 kHz in order to meet EUV source power requirements, these issueswill become even more important since the time between pulses willbecome that much shorter.

FIGS. 1B and 1C show test results from a fourth generation plasma pinchprototype device. FIG. 1B shows the pulse shape on capacitor C2 andacross the electrodes and FIG. 1C shows a measured photo diode signalwith Xenon as the active gas.

High Temperature Electric Discharge EUV X-Ray Devices

The high repetition rate reliable, long-life pulse power systemdescribed above can be utilized to provide high voltage electricalpulses to a variety of extreme ultraviolet or x-ray devices. Thesesystem included a dense plasma focus device depicted in FIG. 2A,conventional Z-pinch device shown in FIG. 2B, a hollow cathode Z-pinchdevice shown in FIG. 2C, and a capillary discharge device as shown inFIG. 2D. In each case the light source is generally symmetrical about anaxis referred to as the “Z” direction. For this reason these sourcesespecially the first three are often referred to as “Z” pinch lightsources.

Dense Plasma Focus

The principal feature of a dense plasma focus EUV device is shown inFIG. 2B. These are anode 8A, cathode 8B and insulator 8C and a highvoltage pulse power source 8D. In this case when high voltage is applieda discharge starts between the cathode and the anode running along theoutside surface of insulator 8C. Forces generated by the high plasmacurrent, forces the plasma generally upward then inward creating anextremely hot plasma pinch just above the center of the anode.

The parameters specified above for the pulse power system shown in FIG.1 were chosen especially for this light source to produce 12 J pulses ofabout 5,000 volts with pulse durations of about 100 to about 500 ns.Preferably a preionizer (which may be a spark gap preionizer) isprovided as described in more detail in U.S. patent application Ser. No.09/690,084 which has been incorporated by reference herein.

FIG. 2A(1) shows a cross-section of a portion of a fourth generationplasma pinch EUV light source actually built and tested by Applicantswhich incorporates the pulse power system described in FIG. 1. Many ofthe electrical components referred to above are designated in FIG.2A(1). FIG. 2A(2) is a blow-up of the electrode region of the deviceshowing in greater detail the anode 8A, the cathode 8B and the spark gappreionizers 138. FIG. 2A(3) is a drawing of the fourth generation deviceshowing many of the electrical components shows in FIG. 2A(1) and alsoshowing vacuum 3.

Conventional Z Pinch

A conventional Z-pinch light source is shown in FIG. 3. In this case thedischarge starts between the anode and the cathode along the insidesurface of insulator 9C. The forces generated by the high-current,forces the plasma to the center of the cylindrical volume formed byinsulator 9C and causes the plasma to pinch with extremely hottemperatures near the upper end of the volume.

The pulse power circuit shown in FIG. 1 with the components describedabove would work for embodiments of the conventional Z-pinch design,although persons skilled in the art may choose to make changes tocoordinate the parameters of the pulse power electrical components withspecific design parameters of the Z-pinch. For example, if 5,000 voltpulses are preferred this can be easily accomplished simply with thesame basic circuit as shown in FIG. 1 but with one additional one-turnprimary winding on the pulse transformer 406. With this design apreionizer is usually provided to help initiate the plasma at the startof each pulse. These preionizers may be spark gap or other preionizersource and are usually powered from a separate source not shown.

Hollow Cathode Z-Pinch

The hollow cathode Z-pinch shown in FIG. 2C is very similar to theconventional z pinch. The difference being that the cathode isconfigured to produce a hollow below the cylindrically shaped insulator.This design can avoid the need for a preionizer because a very largenumber of ions and electrons are naturally produced near the top of thehollow region 9E at the beginning of each pulse when the high voltageincreases to a sufficiently high level. For this reason this design doesnot require a high voltage switch to initiate the discharge. Thedischarge is referred to as having been self-initiated.

When using the power supply shown in FIG. 1 to provide pulse power forthis design, the last saturable inductor L53 could be eliminated or itsvalue reduced substantially since the development of plasma in thehollow in the cathode serves the same purpose as saturable inductor L53of holding off the discharge until the peaking capacitor C2 issufficiently charged, then permitting current to flow substantiallyunimpeded.

This hollow cathode Z-pinch may be designed for significantly higherpulse voltages than the first two designs. This is no problem with thepower supply shown in FIG. 1. A discharge pulses of, for example, 10,000Volts are easily provided by merely increasing the number of one-turnprimary windings of the transformer 406 from 3 to 9.

Capillary Discharge

A drawing of a conventional capillary discharge EUV light source isshown in FIG. 2D. In this design the compression of the plasma createdby the high voltage discharge between the cathode and the anode isachieved by forcing the plasma through a narrow capillary whichtypically has a diameter in the range of about 0.5 mm to 4 mm. In thiscase the pulse duration is in the order of about 0.5 microseconds to 4microseconds as compared to about 100 to 500 nanoseconds for theembodiment shown in FIGS. 2, 3 and 4. Also, the pulse voltages aretypically substantially lower, such as about 1500 volts. However, thesame pulse power system provides an excellent electrical power sourcewith minor modifications. A simple modification is to eliminate the laststep of magnetic compression which is accomplished by leaving off the C2capacitor bank and the LS3 saturable inductor. The peak pulse voltagecould be reduced to 2,000 by windings in pulse transformer 406 fromthree to one, or the transformer could be eliminated with an increase inthe initial charging voltage to provide electrical pulses of a fewmicroseconds and a peak voltage of about 1500 volts.

Laser Produced Plasma

As described in the background section of this specification, a priorart technique for producing extreme ultraviolet light on soft x-rays isto use short pulse lasers to produce a very hot plasmas which aresimilar to the plasmas produced in the plasma pinches described above.Prior art techniques typically utilize solid state lasers such asQ-switch Nd-YAG lasers pumped with diode lasers (or flash lamps) toproduce very high power nano-second or pico second laser pulses whichare focused on a target material which may be the same target materialsas the active materials identified above such as lithium and tin whichproduce debris or xenon which does not produce debris. Some of theseprior art light sources are described in U.S. Pat. Nos. 5,668,848,5,539,764, and 5,434,875, all of which are incorporated herein byreference. These prior art patents teach the use of an Nd-YAG laser forgenerating the plasma and the use of an Nd-YAG seeded XeCl excimerpre-amplified and an XeCL excimer amplifier for producing the high power(such as 1×10¹¹″ Watts) very short pulse laser beam for generatingplasmas in target material. Other laser systems (including excimer lasersystems) have been proposed for producing x-rays (see for example, M.Chaker, et al., J. Appl. Phys. 63, 892 (1988; R. Popil et al, Phys. Rev.A 35, 3874 (1987); and F. O'Neill et al., Proc. SPIE 831, 230 (1987).Applicants have determined that many of the novel features developed byApplicants in connection with Applicants' development of their plasmafocus devices can be applied with respect to laser produced plasmas justas well as plasmas produced by the various pinch devices shown in FIGS.2A-D.

Applicants' employer is the leading supplier in the United States andinternationally of excimer laser light sources for integrated circuitlithography. These lasers are KrF excimer lasers operating at 248 nm andArF lasers operating at 193 nm. These lasers are extremely reliable,typically operating 24 hours per day 365 days per year with up-times, onthe average better than 99 percent. During the past several years pulserepetition rates of these lasers have increased from about 100 Hz in1990 to 4000 Hz in 2003. The average power of these lasers has increasedfrom about 1 Watt in 1990 to about 120 Watts in 2003. The pulse durationis about 20 ns and the current pulse energy is about 30 mJ. Techniquesto increase repetition rates of these lasers to 6,000 to 10,000 Hz aredescribed in U.S. patent application Ser. No. 10/187,336 alsoincorporated herein by reference.

Applicants believe that the excimer laser systems currently in use asthe leading lithography light source at 248 nm and 193 nm can be adaptedto provide extreme ultraviolet light in the range of 11 to 14 nm.Examples of these laser systems are described in the following U.S.Patents and patent applications which are incorporated herein byreference: U.S. Pat. Nos. 6,128,323; 6,330,261, 6,442,181, 6,477,193 andU.S. patent applications Ser. Nos. 09/854,097, 09/943,343, 10/012,002,10/036,676, and 10/384,967.

In a laser-produce plasma light source the laser energy is absorbed bythe inverse Bremsstrahlung mechanism. Due to their shorter wavelength,excimer lasers can couple energy more efficiently to the target plasmathan near infrared or visible laser radiation from (frequency-doubled)solid state lasers. (The plasma frequency and thus the critical densityis higher at shorter wavelength of the pump laser.) Due to their shorterwavelength, excimer lasers can (if desired) be focused more tightly to a(diffraction-limited) spot size than longer wavelength (e.g.,solid-state) lasers. This increases the power density of the source. Theexcimer laser should be a Cymer laser, since these are the most reliableones in the world. If desired several excimer laser beams can becombined in one spot. This permits power scaling.

One or several excimer laser beams are tightly focussed onto a (gaseous,liquid or solid) target inside a vacuum chamber to generate a hotlaser-produced plasma. When the proper target material is used and theright mean electron temperature is reached in the plasma, EUV radiationat 13.5 nm can be efficiently generated. Suitable target materials arexenon, tin and lithium. Xenon has advantages with respect to lowerdebris production. Unfortunately, xenon is not the most efficient targetat 13.5 nm, in particular not for a laser-produced plasma. It producesradiation much more efficiently at around 11 nm. One of the best targetconfigurations is a liquid jet of xenon, since the plasma can begenerated at a fairly large distance from the nozzle. Tin has advantageswith respect to conversion efficiency, since many ionization stagescontribute simultaneously to the 4d-4f emission at about 13.5 nm. Indiumhas advantages, if its corresponding radiation band at 14 nm and aboveis used. (There, the manufactured multi-layer mirrors have only slightlylower peak reflectance but larger bandwidth. Therefore, a higherintegral in-band intensity can be obtained.) Lithium has advantages incase a light source with narrower emission bandwidth should be required,since lithium emits efficiently in a narrow line at 13.5 nm. It may beadvantageous to use a small cavity for confinement, in particular, ifmetals are used as laser targets. Liquid metal targets (molten tin,indium or lithium) offer the possibility of high target density andreproducible target conditions when the source is operated at constantrepetition rate. (A crater will be formed, but a given, fairly constantshape will dynamically evolve at a given repetition rate between thelaser pulses.)

The excimer pump laser should preferably be operated withkrypton-fluoride at 248 nm, since this is the most efficient excimerlaser and since associated optics issues for the focussing optics areless severe. The excimer laser preferably is operated broad-band and ina MOPA configuration, since a very high output power is needed. Thelaser pulse duration should be as very short (a few nano seconds such asabout 20 ns), since it has to be matched to the plasma expansion time.The peak power will be high. In preferred embodiment the laser isoperated at repetition rates of 10 kHz or higher, at least at more than5 kHz. To increase the effective repetition rate, one may also combineseveral lasers operated at suitable different times in the interval.This depends to a large part also on the target configuration andreplenishing rate of the target material. It is advantageous to have atailored laser pulse that is incident on the target. In a preferredembodiment a pre-pulse portion generated for instance by the excimerlaser oscillator (which may bypass the power amplifier in order tominimize the travel time to the target) containing up to a few percentof the total laser energy arrives at the target first to form apre-plasma. This pre-plasma will absorb the main laser pulse much moreefficiently. The pre-plasma can also be accomplished by using adifferent, perhaps smaller-power laser.

The laser beam will be focused by optimized focusing optics mountedimmediately in front or behind of a vacuum window. The objective is toachieve a focal spot of less than about 100 μm diameter. The spot sizedepends to some extent on the laser pulse duration (10 to 30 ns), sincethe plasma expansion velocity has to be taken into account. In short,the laser pulse duration has to be short enough and the spot size smallenough to keep a large portion of the plasma tightly together during themain heating period. Typical expansion times are on the order of 10-100μm per nanosecond.

The laser systems described in details in the above identified excimerlaser patents and patent applications produce a very line-narrowedpulsed laser beams, line narrowed to about 0.5 pm or less. This permitsfocusing to a quarter micron spot. However, these laser system can beoperated broadband in which the bandwidth of the output pulse laser beamis about 35 nm for KrF lasers with the line center at about 248 nm.Broadband operation permits substantial increases in energy of theoutput beam. For example, a KrF MOPA system of the type described inpatent application Ser. No. 10/384,967 could produce 330 mJ pulses (ascompared to the 30 mJ line narrowed pulses). The instantaneous pulsepower for the 20 ns pulses is about 165×10⁶ Watts. According toexperiments performed at Lawrence Livermore Laboratories (J. Appl. Phys.79(5): March 1996) using a Nd/YAG laser, the maximum EUV output occurredat a laser intensity of 2×10¹¹″ W/cm². While maximum conversionefficiency (EUV energy output/laser energy input) occurred at about2×10¹¹″ W/cm². These experiments indicated not much variation in resultswith changes in wavelength. The pulse duration in the experiments werenot much different from the 20 ns pulses of Applicants employers'excimer lasers. For the 165×10⁶ Watt pulses therefore Applicants preferspot sizes in the range of about 0.1 mm² which would provide intensitiesof about 1.6×10⁻¹¹ W/cm² which is in between maximum efficiency andmaximum output.

The energy of the laser pulse is about 330 mJ so at a conversionefficiency of about 0.006 the EUV pulse energy is about 2 mJ/pulse. At6000 Hz this corresponds to an EUV production of about 12 Watts. About20 percent of this light can be collected and delivered to anintermediate focus such as location 11 in FIG. 19 using technologydescribed herein. So the average in-band EUV power from one excimerlaser produced plasma delivered to the intermediate focus is about 2.4Watts. The combination of two systems would produce about 5 Watts. Insome applications, this is sufficient.

Applicants have been told that makers future EUV lithography machineshave desires for an EUV light source of about 45 Watts to about 100Watts at an intermediate focus such as location 11 in FIG. 19. But thisrequirement is for several (at least 5) years in the future and therequirement is contingent on development of corresponding lithographysystems that can handle EUV power in the 100 Watt range. Since theexcimer laser can be expected to couple energy more efficiently to theplasma (shorter wavelength, higher critical density) than a solid-statelaser driver at 1.06 μm, the conversion efficiency should be higher forthe excimer-laser produced plasma as compared to the prior art NdYAGlaser.

About 10 kW of laser power will be needed to generate the required EUVpower of about 100 Watt at the intermediate focus of the lithographytool. With expected improvements in demonstrate conversion efficiencies,each KrF module (broad-band operation at 248 nm) can be expected toprovide about 1 to 1.2 kW of laser power (e.g. 6 kHz repetition rateoperation at 200 mJ/pulse). A total of nine such modules would deliverthe required laser power. More than 200 W of in-band EUV radiation wouldbe produced at the source (2% bandwidth into 2π) and about 100 W in-bandEUV could be collected and delivered to the intermediate focus.

There are different ways to combine the laser beams (multiplexing).Laser beams can be (nearly) overlapped optically by mirrors and lasersbeams can be focused through the same lens from slightly differentdirections onto the same focal spot. The lasers can also be triggered ina staggered fashion such the effective repetition rate is increased,provided the target is replenished fast enough that it can sustain thehigh repetition rate. For instance, tripling of the repetition rate withthree laser systems to about 18 kHz seems feasible.

FIG. 4 shows one embodiment where the laser beams from several lasermodules can be aimed at different portions of the focusing lens and madeto spatially overlap in the common focus which corresponds to thelocation of the laser-produced plasma. The emitted EUV radiation iscollected over a large angular range by the multi-layer coated firstcollector mirror and directed to the intermediate focus.

FIG. 4A shows another embodiment where the laser beams from severallaser modules are overlapped in a common laser focus with separatefocusing optics for some of the laser beams. The laser radiation can befocused through several openings in the first collector mirror. Thisembodiment makes use of the fact that the EUV radiation generated fromthe laser plasma has an angular distribution that is peaked to someextent in the direction of the incoming laser beam (and weaker at anglesorthogonal to the laser beam). In this embodiment, the regions ofstrongest emission are not blocked by the space required for the beamdelivery device.

Target Delivery

The preferred target for the laser plasma is a so-called mass-limitedtarget. (Just the right amount necessary for the laser-produced plasmais provided, no more, in order not to increase the production of debrisunnecessarily. For xenon, a preferred target technique is a thin liquidjet. Cluster beam targets and spray targets may also be employed usingerosion resistant nozzles. For metals (tin and indium), liquid metaldrops, immersed in a helium beam, are suitable. A nozzle, mounted fromthe top, and a target beam dump mounted below, comprise a suitablesystem. See FIG. 4B. The plasma-facing surfaces may be coated by athermally conductive thin film, like carbon or diamond coating, toreduce erosion, since ion sputtering is reduced.

Laser Plasma Supported EUV Pinch

The laser plasma source has the advantages of high source brightness(small source volume), no erosion, less debris generation. It has thedisadvantages of high cost-of-ownership and inefficient total energyconversion balance. The discharge source has the advantages of directcoupling of the electrical energy into the pinch plasma and ofsimplicity. It has the disadvantages of electrode erosion and highdebris production, as well as thermal management issues.

The laser beam(s) and the laser plasma are used to define the plasmageometry, discharge pathways and plasma pinch location. The arrangementis such that there is a larger distance from the electrodes to theplasma focus than in a pure discharge source. This reduces the powerdensity at the electrode surfaces, since they can be large, and thusalso electrode erosion, debris generation and thermal management risksare reduced. On the other hand, the main power input is provided by thelow-inductance electrical discharge. This ensures a much more efficientenergy coupling to the plasma than would be available for a pure laserplasma source. The arrangement of the electrodes is more spherical thanfor a conventional Z-pinch. This and the laser-plasma initiationincrease the source stability. The timing of pre-ionization, laserplasma generation and main pinch plasma generation gives additionalcontrol for optimization of the production of EUV radiation.

The device is mainly a discharge-produced EUV light source that has theadditional benefits of laser-plasma supported discharge initiation. Theelectrodes can be connected to the same pulsed-power system that is usedpresently (and in the future) for the DPF machines. (10 J to 20 Jdelivered pulse energy, 30-100 ns pulse length, repetition rate ofseveral kHz, peak voltage of several kV, peak current several tens ofkA.) The inner electrode can be charged positive or negative. The outerelectrode is at ground potential. As shown in FIG. 4C the electrodearrangement is somewhat different from the DPF arrangement. The(water-cooled) electrodes are bigger and the electrode surface involvedin the discharge is bigger. It is on the order of 30 to 50 cm². There isan insulator disk between the electrodes to prevent a discharge alongthe direct line-of-sight.

There is a means of pre-ionization, e.g., pulsed RF- pre-ionization viaRF-coil. The pulsed laser beam (excimer laser or solid-state laser) thatpropagates on-axis is focused by a focusing optics into the center ofthe arrangement to a focal spot with a diameter of ca. 100 μm. The lasercan be a KrF-broad-band excimer laser with 100 mJ to 200 mJ pulseenergy, about 10 to 15 ns pulse length and several kHz repetition rate.There could also be several laser beams focused into a common spot inthe center of the arrangement. The target gas, xenon or a mixture ofxenon and helium, is inserted from inside of the inner electrode and ispumped away by a vacuum pump. Typical operating pressure is in the rangeof 1 to 0.01 Torr. The discharge can be operated on the left side of thePaschen curve. If the inner electrode is pulsed-charged by a negativehigh voltage, it can be configured as a hollow cathode.

First, RF pre-ionization is triggered to enable easy breakdown of thelow-density gas. Next, the laser beam arrives and generates awell-defined plasma spot at the center of the arrangement. The gasbreaks down near the laser focus, since it was pre-ionized. Then themain discharge from the pulse-compression circuit is applied. A pinchwill develop on-axis at the laser-plasma spot. Pinching occurs bymagnetic self-compression. The laser-plasma spot defines the location ofthe pinch and increases its positional stability. (In case theinductance in the center should be too high, the laser beam needs to bedoughnut-shaped in order to provide a discharge channel. This has to betested experimentally.) The expanding shock front from the laser plasmawill encounter the radial compression front from the main pinch plasmawhich is stronger. A pinched plasma channel develops which will heat thegas to high ionization levels that will emit the EUV radiation. Thecounter-propagation of the two plasma shock fronts can effectivelyincrease the duration of the pinch and thus the duration of the EUVemission. The EUV radiation is emitted in all directions. The radiationemitted through the large opening of the outer electrode can becollected by grazing-incidence collection optics. The energy, the sizeof the focus and the timing of the laser plasma determine the size ofthe main pinch plasma.

Radiation Collector Materials

The radiation produced at the radiation spot is emitted uniformly into afull 4π steradians. Some type of collection optics is needed to capturethis radiation and direct it toward the lithography tool. Severalmaterials are available with high reflectivity at small grazing incidentangles for 13.5 nm UV light. Graphs for some of these are shown in FIG.11. Good choices include molybdenum and rhodium in the range of 0 toabout 20 degrees and tungsten for very small grazing angles. Thecollector may be fabricated from these materials, but preferably theyare applied as a coating on a substrate structural material such asnickel. This conic section can be prepared by electroplating nickel on aremovable mandrel.

Conical Nested Collector

To produce a collector capable of accepting a large cone angle, severalconical sections can be nested inside each other. Each conical sectionmay employ more than one reflection of the radiation to redirect itssection of the radiation cone in the desired direction. Designing thecollection for operation nearest to grazing incidence will produce acollector most tolerant to deposition of eroded electrode material. Thegrazing incidence reflectivity of mirrors such as this depends stronglyon the surface roughness of the mirror. The dependence on surfaceroughness decreases as the incident angle approaches grazing incidence.Applicants estimate that their devices can collect and direct the 13 nmradiation being emitted over a solid angle of least 25 degrees.

In another preferred embodiment the collector-director is protected fromsurface contamination with vaporized electrode material by a debriscollector which collects all of the tungsten vapor before it can reachthe collector director 4. FIG. 9 shows a conical nested debris collector5 for collecting debris resulting from the plasma pinch. Debriscollector 5 is comprised of nested conical sections having surfacesaligned with light rays extending out from the center of the pinch siteand directed toward the collector-director 4.

The debris collector collects vaporized tungsten from the tungstenelectrodes and vaporized lithium. The debris collector is attached to oris a part of radiation collector-director 4. Both collectors may becomprised of nickel plated substrates. The radiation collector-directorportion 4 is coated with molybdenum or rhodium for very highreflectivity. Preferably both collectors are heated to about 400° C.which is substantially above the melting point of lithium andsubstantially below the melting point of tungsten. The vapors of bothlithium and tungsten will collect on the surfaces of the debriscollector 5 but lithium will vaporize off and to the extent the lithiumcollects on collector-director 4, it will soon thereafter also vaporizeoff. The tungsten once collected on debris collector 5 will remain therepermanently.

Parabolic Collector

FIG. 8C shows the optical features of a collector designed byApplicants. The collector as shown in FIG. 8A is comprised of fivenested grazing incident parabolic reflectors, but only three of the fivereflections are shown in the drawing. The two inner reflectors are notshown. In this design the collection angle is about 0.4 steradians. Asdiscussed below the collector surface is coated and is heated to preventdeposition of lithium. This design produces a parallel beam. Otherpreferred designs would focus the beam. The collector preferably iscoated with a material such as those referred to above and graphed inFIG. 11 possessing high glazing incidence reflectivity in the 13.5 nmwavelength range.

Ellipsoidal Mirror

Another collector-director designed to focus the beam is shown in FIG.8. This collector-director utilizes an ellipsoidal mirror 30 to focusthe EUV source. Mirrors of this type are available commercially fromsuppliers such as Reflex S.V.O. with facilities in the Czech Republicand are distributed in the United States by Bede Scientific InstrumentsLtd. with offices in the United Kingdom and Englewood, Colo. The readershould note that this mirror collects only rays at angles shown at 32 inFIG. 8. However, additional mirror elements could be included insidemirror 30 and outside mirror 30 to collect and focus additional rays.The reader should also note that other mirror elements could belocalized downstream of mirror 30 to collect the narrow angle rays orupstream of mirror 30 to collect the wider angle rays.

Tandem Ellipsoidal Mirror

FIG. 19 shows a preferred collector director design for greatlyimproving the EUV beam profiled. This is a tandem ellipsoidal mirrorunit which collects and directs the EUV radiation produced in the plasmapinch.

In most lithography applications the target region needs to be exposeduniformly. A single or nested ellipsoidal mirror of the type shown inFIG. 2A when used to collect and re-focus the EUV radiation produces avery non-uniform annulus of radiation upstream and downstream of focusspot 11 shown in FIG. 2A. This is a natural effect caused by thegeometry of the ellipsoidal collector. The front of the mirror collectsa greater solid angle of the source emission per unit mirror surfacearea than the back of the mirror. This effect can be reversed by using asecond ellipsoidal mirror 44 in tandem with the first mirror 42 as shownin FIG. 19. (In this embodiment, single ellipsoidal mirrors are usedwithout a second nested ellipsoidal mirror.) The second ellipsoidalmirror 44 is a mirror image of the first ellipsoidal mirror 42“reflected” about the second focal point of the first mirror. Thisplaces the second ellipsoidal mirror on the same optical axis as thefirst mirror so that its first focal point is at the second focal pointof the first mirror. In this case of the tandem ellipsoidal mirror theradiation leaving the second focal point of the second mirror is annularbut the radiation within the annulus is uniform. The exposure uniformityis now a function of the surface figure of the ellipsoidal mirrors andnot the inherent collection geometry of the ellipsoidal mirror.

Analysis

The optical characteristics of the tandem ellipsoidal mirror wereanalyzed by Applicants with the ray tracing code, TracePro, supplied byLambda Research Corporation of Littleton, Mass. The EUV radiation fromthe DPF source is incoherent. Consequently, a ray tracing code can beused to determine the properties of the radiation collected and leavingthe tandem mirror. The EUV radiation requires special reflectivesurfaces such as molybdenum or ruthenium. This analysis was performedunder the assumption that the mirror surface has a perfect ellipsoidalreflector and that the radiation is not polarized during reflection. Themirror surface was assumed to be pure ruthenium reflecting at 13.5 nm.Also, the source has been assumed to be a 50 micron diameter disc andthat the radiation emits isotropically from each point on its surface.These assumptions do not detract from the basic ability of the tandemmirror to produce a uniform annular exposure region.

The geometry of the tandem ellipsoidal mirror is illustrated in FIG. 19.Both mirrors have the same parameters. Their minor radius is 40 mm andtheir focal length is 150 mm. The mirrors are each 100 mm long and havebeen cut through their minor diameter. The figure also shows a fewrandom rays collected by the first mirror. A fraction of the radiationthat leaves the plasma pinch source 46 at the first focal point of thefirst mirror is collected and re-focused at the second focal point 11 ofthe first mirror. The radiation leaving focal point 11 at 300 mm fromsource 46 is collected by the second ellipsoidal mirror and re-focusedat the second focal point of the second mirror 48 at 300 mm from focalpoint 11. At focal point 48 a 1:1 image of the source is produced. Asthe radiation leaves focal point 48, the rays diverge to produce anannular exposure area at detector 50 which is located 9 mm from focalpoint 48. The intensity in this annular region is uniform as shown bythe TracePro calculation in FIG. 19. The uniformity in the main annularregion is within ±2.5% of the mean value. A simulation performed byApplicants of the beam profile at detector 50 is shown in FIG. 19 whichmay be compared with a similar simulation made for the beam crosssection at 9 mm downstream of focal point 11. A cross section of the twoprofiles is compared in FIG. 19 with the detector 50 cross section shownat 52 and the cross section of the FIG. 19 beam profile at 54.

Fabrication

The techniques for ellipsoidal mirror fabrication have been improvedover the past few 10s of years. The surface quality of these mirrors cannow be made to satisfy the requirements of surface figure, surfaceroughness, and the material of the reflecting surface for their use inthe EUV region. Four materials have been identified as possiblecandidates for the EUV ellipsoidal mirror surface: molybdenum,ruthenium, rhodium, and palladium. These materials have relatively highgrazing incidence reflectivity at 13.5 nm. The grazing incidencereflectivity must remain high at relatively high angles to allow themirror to collect a reasonable solid angle subtended from the source.Theoretically, ruthenium has the highest collection efficiency of thefour materials listed.

These mirrors are fabricated though a series of processes. First, amandrel is made that has the outside figure of the desired mirror.Typically, the mandrel is made undersize using aluminum and then coatedwith electroless nickel containing 15% phosphorus to make the mandreloversize. The electroless nickel is put on about 0.5 mm thick so thatthe entire surface can be diamond turned to the desired mirror surfacefigure by vendors such as Corning Netoptic with offices in Marlborough,Mass. This typically leaves about 0.1 mm of nickel on the mandrelsurface. Although the present technology of diamond turning is very goodthe surface at this stage is not adequate for use as an EUV mirror. Thediamond turning can be accurate enough for the figure requirements thatinclude the deviations from the elliptical surface front-to-back and theroundness of the surface but the micro-roughness is too high. Thediamond turned surface must be polished to reduce the micro-roughness toless than 0.5 nm RMS. The hardness of the nickel surface imparted by thehigh phosphorus content of the electroless nickel is required for thehigh degree of polishing. After the electroless nickel surface isadequately polished and the surface figure is within specifications, thereflecting surface material is coated onto the mandrel surface. Theexact procedure used to coat the surface is dictated by the propertiesof the reflecting material being added to the surface. After thereflecting coating has been placed on the mandrel, nickel iselectroformed over this surface to a thickness of about 0.5 mm. Theelectroformed nickel is removed from the mandrel by applying force alongthe axis of the mandrel between the mandrel and the electroformednickel. The reflecting surface stays with the electroformed nickel shellto form the mirror as it slides off the nickel surface on the mandrel.The surface of the highly polished electroless nickel with the highphosphorus content acts as a natural release agent for the reflectingsurface. After the mirror has been removed from the mandrel and themandrel re-polished, the mandrel is then available to make additionalmirrors that are exact copies of the first mirror.

Alignment

The positioning of the mirrors relative to the source and to each otheris critical to the correct function of the tandem ellipsoidal mirrors.Alignment can be accomplished on an optical bench with a source placedat the same location as the DPF EUV source. One must take advantage ofthe optical properties of these ellipsoidal mirrors. If a detector planeis placed perpendicular to the optical axis near the second focal point,the small source, 50 microns diameter, e.g., can be placed near thefirst focal point of the ellipse. The image will only be centered andsymmetric if the detector is at the second focal point. After the axiallocation of the second focal point has been determined, the detectorarray can be moved away from the focal point. Now the image will only besymmetric if the source is on the mirror axis. This requires positioningthe source in two spatial dimensions. The axial location of the firstfocal point can be determined by moving the detector to the second focalpoint and then moving the source along the mirror axis until thedetector gives a maximum signal in the image center.

This procedure must be repeated for the second mirror. After the twomirrors have been aligned, the entire assembly must be transferred tothe DPF. The fixture must be adequately keyed to place the EUV source atthe first focal point of the first mirror. The accuracy of positioningmust be at least 25% of the effective diameter of the DPF EUV source.The present estimate of the DPF source diameter is 80 microns whilelooking along the machine axis. Hence, the expected alignment accuracyis 20 microns in the plane perpendicular to the machine axis. The axialalignment of the tandem mirror is not as critical and is expected to beabout 0.5 mm.

Lithography Projection Optics

The EUV projection in preferred embodiments is designed to map thesource spot into the entrance pupil of the projection optic and to mapthe far field intensity (i.e. the energy vs. angle) of the source ontothe reticle. Such designs are desirable because the uniformity in theentrance pupil, though important, is not critical while the uniformityat the reticle plane is critical. This design concept exploits the factthat the emission is isotropic and thus has uniform intensity vs. angle.The dual mirror concept restores this uniform intensity vs. angleproperty (at least within the cone of capture angle for the mirrors).The EUV illuminator take the “ring” of intensity versus angle, break itinto pieces or arcs, and overlay these arcs onto the reticle. Thisfurther improves the uniformity and can be done in EUV systems sincethey are scanners and thus require illumination only over a slit region.

Debris Mitigation

Both the mid-focal point 11 between the two mirrors and the final focalpoint 48 allow the DPF source region to be isolated from the lithographyexposure region. At these points the EUV radiation can pass throughpinholes that block any source debris or active gas (that penetratedinto the region of the first elliptical mirror unit) from reaching theexposure chamber but not the EUV radiation. Also, these small pinholesallow the exposure chamber to have a much lower pressure than thatrequired for DPF operation.

Hybrid Collection

Based on currently available reflector technology, only two types ofreflectors exist which provide reflection values in the 0.7 or greaterrange for this 12-14 nm EUV light. As shown in FIG. 11A a few materialsprovide good grazing angle reflectors. For example, reflection fromsmooth molybdenum surfaces is about 90% grazing angles less than 10degrees, but reflection from molybdenum drops rapidly at grazing anglesgreater than 15 degrees to less than 10% at 25 degrees. On the otherhand, special multi-layer reflectors have been designed that providereflectivity values in the range of 60% to 70% at normal incident anglesbut the reflectivity of these multi-layer reflectors remains high foronly about 5-8 degrees from normal and drops to less than about 10% atincident angles greater than about 10 to 15 degrees. Other multi-layermirrors can be designed for about 30 percent reflectivity over a broaderrange up to about 20 degrees around normal. Using these available mirrortechnologies Applicants have developed various collector designs formaximizing the collected light. Three of these designs are shown inFIGS. 11B, 11D and 11E. Applicants refer to these collectors as hybridcollectors since they utilize multiple collection designs. For example,the prior art includes nested elliptical mirrors and nested grazingangle by hyperbolic mirrors including double bounce hyperbolic mirrorsand most multi-layer reflector designs are single bounce near normalhyperbolic designs. FIG. 11B is a partial cross-section of a hybridcollector utilizing two ruthenium coated ellipsoidal mirrors 80 and 81and two double bounce ruthenium coated parabolic mirrors 82 and 83 toprovide a 1500 mm focal length. FIG. 11C shows the reflectionefficiencies of the mirrors at the angles of incident of the lightbetween about 10 degrees and 55 degrees. This design collectssignificantly more light than prior art elliptical designs or prior arthyperbolic designs. Applicants estimate that about 25 percent of theemitted light is collected and 79 percent of the collected light isdelivered to the intermediate focus at 1500 mm. This equates to anestimate 20 percent collection efficiency.

FIG. 11D shows a modified version of the FIG. 11B collector in which anadditional parabolic double reflection mirrors 84 and a parabolic triplereflection mirror 85 are utilized to increase the net energy collectedto about 28 percent.

FIG. 11E shows a third hybrid version also a modification to the 11Bcollector which (in addition to the two ellipsoidal reflectors) and thetwo-bounce parabolic reflectors, Applicants have added a thirdtwo-bounce parabolic mirror 86 and a grazing angle curved ray-tracedmirror 87 and a multi-layer parabolic mirror 88 reflecting at about 9degrees from normal to increase the collection efficiency from about 20%to about 25%.

In another embodiment, a multitude of laser beams can be focused throughcorresponding openings of the electrodes to a common central focal spot.The main discharge follows along the laser channels and converges ontothe center plasma.

Debris Shields Techniques for Making Debris Shield

As described above debris shields are important elements insubstantially all EUV light sources now under consideration. The perfectdebris shield won't trap all debris and transmit all in band radiation.Since the debris shield will likely have a limited lifetime, it shouldalso preferably not be difficult to make. Three preferred techniques forfabrication debris shields are shown in FIGS. 28A-B, 29A-C and 30A-C.

For the technique described in FIGS. 26A and B, removable skinny pyramidshaped forms as shown in FIG. 26A are fabricated and the small end ofthe forms are inserted in a grid shaped structure such as the one shownin 28B. A spacer plate with tabs matching a hole at the large end ofeach of the forms is placed over the larger end of the forms to separateeach form from other forms by the thickness of the grid which preferablyis about 0.01 to 0.1 mm or less. The grid spacing provides a narrowspace between the forms which is filled with a liquid metal or liquidceramic. When the metal or ceramic has hardened the forms are removed tocreate the debris shield.

For FIGS. 5A-C technique, hollow cones such as those shown 76 in FIG. 5Bare welded from very thin about (0.1 mm) metal foil cut from foil sheetsas shown at 77 in FIG. 5A. These hollow cones are inserted into a metalform as shown at 78 in FIG. 5C to form the debris shield.

As shown in FIGS. 7A-C, a preferred debris shield can be made bylaminating thin sheets. Each sheet has its own individual radial grillework with grille work patterns growing larger for each sheet so thatwhen multiple sheets are stacked the desire shape is produced as shownin FIGS. 7A-C.

An advantage of the laminated approach is that the uneven surfaces ofthe channels provides a torturous path for particulate with multipleeddys for particulate to collect within. Another advantage is that theshield assembly can be constructed of multiple materials. It may provebeneficial to use heat resistant ceramics close to the light source, orperhaps materials with excellent thermal conductivity such as copperthat can assist in removing heat from the same region.

Magnetic Suppression

Another technique for increasing the effectiveness of debris shields inthese EUV light sources is to apply a magnetic field in the region ofthe debris shield and the region between the pinch and the shield. Themagnetic field preferably is directed perpendicular to the axis of theEUV beam so as to force charged particles into a curved trajectory as itapproaches and passes into the debris shield. To enhance theeffectiveness of the debris shield the debris can be further ionizedpost pulse. This can be done with the same components used forpreionization or similar ionization components could be used for thepost pinch ionization.

In another embodiment a coil with large diameter (larger than thecollector mirror diameter) will be mounted co-axially with the mirrorand plasma source. Gneerally, a high current will be applied to the coilto induce a high magnetic field in the axial direction. Preferentially,the current may be pulsed (pulse width on the order of several 10 μs) toachieve a high induction field strength (on the order of 10 Tesla).Constant fields and preferentially super-conducting coils may also beemployed to generate these high fields. This is sufficient to deflectmost energetic ions to curved paths, such that they miss the collectormirror. The high magnetic filed will lead to a slight elongation of theplasma source volume, but this can be tolerated. The coil has to bemounted on some support structure. It is conceivable, to mount the coilinside or outside of the vacuum chamber.

The radius of the curvature of a charged particle in a magnetic field isgoverned by the equation of motion:F=q(vxB)

From which we can derive that the magnetic rigidity (B*R) for an ion ofmass M, accelerated to a voltage V is given by:B*R=144(M*V)^(0.5)

Applying this case where we want to deflect a singly charge Xe ion (mass132) accelerated to 1000 Volts we get a rigidity of:B*R=144(132*1000)^(0.5)(G-cm)=52,318 G-cm

Therefore if we want the ion to move in a circular orbit of radius 10 cmwe require a magnetic field of 52,318 G-cm/10 cm which is equal to ˜5232gauss.

In general to deflect ions of different masses and energies we mayrequire stronger or weaker fields. The configuration of the magneticfield can also be adjusted to optimize the shielding power for the EUVoptics by winding coils in various configurations or using combinationsof coils and permanent magnets to achieve the desired field profiles.For these fields a coil can be placed either outside the vacuum vesselor interior to it. The current driving a coil required to produce agiven magnetic field can be easily calculated.

Honeycomb Debris Shield

FIGS. 9A, 9B, and 9C show examples of a special preferred embodimentutilizing a tapered powder-formed cellular honeycomb body as the debriscollector with an ellipsoidal radiation collector. The debris collectoris preferably produced utilizing one of the techniques described in U.S.Pat. No. 6,299,958 which is incorporated by reference herein. The debrisshield is produced through a reforming procedure wherein a precursorhoneycomb, shaped from a plasticized powder batch material, is filledwith a compatible plastic filler material and then shaped by forcing thefilled honeycomb through a conical shaped form. The process forces ashrinkage of both the filler material and the honeycomb structure. Thestructure now conical shaped is then removed from the form and thefiller material is removed by a process such as melting it. Then the nowconical-shaped honeycomb is then hardened such as by sintering. FIG. 9Ais a three-dimensional cutaway drawing showing pinch region 100,honeycomb debris shield 102 and a portion of ellipsoidal shapedradiation collector-director 104. FIG. 9B shows a cross-section view ofthe FIG. 9A components along with ray traces 106A, B, C and D of four ofthe rays from pinch region 100. FIG. 9C shows how additional ellipsoidalelements can be nested to focus more of the light. Preferably 9 or 10elements are nested within the outside ellipsoidal element. The powders,binder material and filler material can be chosen from the ones listedin the U.S. Pat. No. 6,299,958. The choice of material should be maderecognizing the need of the debris shield to withstand intense extremeultraviolet radiation. A preferred choice is powder and other materialselected to produce cordierite comprised of silicon manganese andaluminum.

Active Materials and Buffer Gas Choice of Active Materials and BufferGases

Several active materials and buffer gases are available for generatingEUV light in the wavelength range of about 13.2 nm to 13.8 nm. Preferredactive materials are xenon, tin or lithium. These three active materialsare discussed above in the section entitled, “Sources for 12-14 nm EUV”.Indium, cadmium and silver are also possible candidates. If one of theabove materials are used as the active material than a noble gas such ashelium neon or argon should be used as the buffer gas. Nitrogen andhydrogen could be added to the potential list of buffer gases especiallyif xenon is the active material. The active materials which are metalsare in most embodiments added to the discharge chamber as vaporsalthough they could be added as liquids or solids and may be added inthe form of a solution or powder.

All of these active materials are chosen because they provide anemission line in the desired range of 13.2 to 13.8 μm and as explainedabove, this is because reflective optics are available with relativelygood properties for UV light in this range. If and when good opticalcomponents become available in other wavelength ranges lower or higherthan this range, then the periodic table and corresponding emission lineliterature should be searched for alternative active materials. Also,buffer gases are not limited to the ones set forth above.

Injection Through Anode

FIG. 18A shows features of a preferred embodiment of the presentinvention in which the active gas in this case Xe (mixed 1 part and 14parts with helium) is injected through the anode. The buffer gas (inthis case 100% He) is injected at 12 in the region downstream ofcollector-director 8. Debris collector 6 comprises nested conicalsections providing narrow passageways in line with rays extending fromthe center of the pinch region to collector-director 8. Thesepassageways permit about 85% of the photons directed towardcollector-director 8 to pass but retards substantially the passage ofdebris generated in the pinch region which follows paths much morerandom than the EUV light. Gas is exhausted from vacuum chamber 10through port 14 by a 40 liter per second vacuum pump. Therefore, buffergas flow from gas feed 12 through the narrow passageways in debriscollector 6 further retards the passage of debris from the pinch andalso retards flow of the Xe active gas from the pinch region into theregion of chamber 10. Therefore, substantially all of the debris fromthe pinch region and active gas injected through port 24 is eitherexhausted through port 14 or coats the surfaces of the debris collectoror the inside walls of the vessel upstream of the debris collector. Thisavoids contamination of collector-director 8 by debris from the pinchand minimize attenuation of the beam by xenon gas since the flow ofbuffer gas through the narrow passageway in debris collector 6 preventsany significant quantity of xenon from entering the region downstream ofdebris collector 6.

Two Direction Gas Flow

FIG. 18B shows features of an embodiment of the present invention inwhich two directional gas flow is utilized to permit a controlledconcentration of active gas near the pinch region with minimumconcentration of active gas in the downstream portion of the EUV beampath. In this case the active gas is introduced through the center ofanode 18A as shown at 24 FIG. 18B. In this preferred embodiment, theintroduced gas is a 1/15 to 14/15 mixture of xenon and helium. Helium isalso introduced at 12 as in the above embodiment. The introduced gasfrom both sources is exhausted at 14 with a vacuum pump of the typedescribed above. Gas flows are controlled to produce a pressure of about0.75 torr in the pinch region and a pressure of about 1 torr in thecollector-director region so that gas flow from the collector directorregion is much greater than the flow from the pinch region.

Upstream Injection of Active Gas

FIG. 18C shows another preferred technique for controlling debris andthe active gas and minimizing EUV absorption by the active gas. Gaspressure in the pinch region is about 0.5 torr. In this embodiment, gasflows within vacuum chamber 10 are arranged to help deter debris fromthe pinch region from reaching the region of collector director unit 8and to minimize the quantity of active gas in the region beyond theimmediate volume surrounding the pinch region. The active gas whichcould be, for example, xenon is injected about 3 centimeters upstream ofthe pinch region through nozzle 2 at a rate of about 5 SCCM and almostall of it is exhausted via a exhaust port 3 running through electrode18A along its axis at a pumping speed of 50 liter/second. The exhaustflow is provided by a vacuum pump such as design blower backed by anAnect Iwata ISP-500 scroll pump available from Synergy Vacuum a Canadiancompany. This provides a pump speed of 40 liters per second. The xenonis fed into nozzle 2 through gas pipe 4 running through the centralregion of debris catcher 6. Debris catcher 6 is comprised of nestedconical sections at 6A having surfaces aligned with light rays extendingout from the center of the pinch site and directed toward collectordirector 8. These nested conical sections provide a relativelyunobstructed passageway for EUV photons produced in the pinch which aredirected toward collector director 8. The passageways are narrow andabout 10 cm long.

Debris collector 6 collects (by condensation) tungsten vaporized fromtungsten electrode 18A. (If the active gas is lithium vapor, the vaporwill also condense on the surfaces of debris collector 6.)

Buffer gas which in this embodiment is helium is injected downstream ofcollector director 8 as shown at 12 and most of the buffer gas isexhausted from vacuum chamber 10 through exhaust port 14 by a vacuumpump (not shown) of the type described above. About 90 percent of thehelium flow passes through collector director 8 in the direction towardthe pinch region and all of the buffer gas passes through the nestedconical section region 6A. As in the above example, this gas flow helpsdeter debris produced in the pinch region from reachingdirector-collector 8 and also minimizes the amount of active gas in thepath of the light being collected and directed by collector-director 8to produce the output EUV beam. These features are important because anydebris accumulation on the surfaces of debris collector 8 reduces itsreflectivity and active gas in the EUV beam path will attenuate thebeam.

Gas exhausted through port 3 is preferably filtered and exhausted to theatmosphere. Gas exhausted through port 14 may also be exhausted to theatmosphere without excessive gas cost since total helium gas flow inthis system is only about 16 grams per hour. Alternatively, the heliumand/or the active gas may be separated and recirculated.

Lithium As Active Gas

Lithium vapor may more efficiently convert the pinch energy into usefullight at the desired wavelength range. Lithium is a solid at roomtemperature and a liquid between the temperature of 180° C. and 1342° C.Many methods are available to introduce lithium vapor into the dischargeand pinch regions. Lithium can be heated to its vapor temperature andintroduced as a vapor. It could be introduced as a solid or liquid andvaporized by the discharge or the pinch or it could be vaporized withother forms of energy such as a high power laser pulse or by some otherform of heating such as a resistance heating element, an electricdischarge or rf heating. Lithium can also be introduced as a compoundsuch as Li₂O, LiH, LiOH, LiCl, Li₂CO₃, LiF, CH₃OLi or their solutions inwater or other liquid.

Lithium may also be delivered to the pinch region by means of laserinduced evaporation or ablation. Lithium metal target 30 will beattached to a holder mounted from the central disk in the debriscollector as shown in FIG. 18D. In one preferred example, a KrF excimerlaser 32 produces a pulsed laser beam of 248 nm wavelength and energy of100 mJ to 200 mJ per pulse, with effective pulse length of 50 ns ispassed through a window 34 mounted on the upstream side of the anode.The light will pass through the hollow anode and be focused by means ofa lens 36 mounted external to the vacuum chamber to a spot ofapproximately 1 mm in diameter. This laser intensity and spot size issufficient to heat the Li metal at such a high rate that the temperaturerise is dominated by the latent heat of vaporization. The thresholdpower density required is about 5×10⁷ W/cm². At lower power Li can alsobe evaporated at a rate governed by its vapor pressure at a giventemperature.

In an alternative embodiment the central region of the central electrodeas shown in FIG. 18A is packed with Li metal as shown at 38 in FIG. 17and the laser beam is passed through the center of the debris shield 8as shown at 40 in FIG. 17.

In another technique by which we can deliver Li to the pinch region isto attach the Li metal to a tungsten plate which is in turn mounted on ahousing containing a permanent magnet. This arrangement is mounted on aninsulating shaft from the debris collector. Li metal is further coveredwith a tungsten mask to expose only a small region of Li. A radiofrequency produced plasma is generated in the region in front of the Litarget by means of an RF generator operating at a frequency of 500 MHzto 2.45 GHz. The discharge may be operated in either pulsed or CW mode.In pulsed mode, the discharge will be synchronized with the plasmapinch. An RF power of 5000 W is generally sufficient.

The generated plasma will be composed of the buffer gas, generally He.He ions will be extracted from the plasma by application of a negativebias voltage onto the Li target. A bias of 500 V to 2000 V will besufficient. He+ ions striking the Li will sputter Li atoms from thesurface. Sputter yields over the bias energies mentioned vary fromapproximately 0.2 to 0.3 for normal incidence. Significantly higheryields can be expected for grazing incidence and for Li at elevatedtemperature.

Preionization Improvements

The DPF can be preionized with a variety of different techniques each ofwhich have a beneficial effect on EUV output. The technique originallyused in Cymer DPF is based on driving a set of spark plug type pins 138mounted in the outer electrode of the device as shown in FIG. 2A(2).These pins can be driven by a high voltage pulse such as the RFsimulator, or by the unipolar output of the 6000 series commutator. Thevoltage required to initiate breakdown using the RF simulator orcommutator is +/−20 kV. Applicants have also demonstrated that thepreionization source can be located remote from the cathode but insidethe main vacuum vessel. This is a coiled antenna. Applicants have alsosuccessfully used a straight antenna for preionization.

This type of antenna can be either linear or shaped in the form ofhelical coil. The antenna can be driven either by an RF simulatordelivering high voltage (such as about) pulses at 13 MHz for 2 μs, thecommutator delivering either a positive or negative polarity pulse or byan RF amplifier. We have demonstrated to support (10 kHz pulserepetition rate). External preionization (antenna located outside of theanode/cathode region) has been shown to be the desireable mode ofpreionizing the negative polarity deep plasma focus. With positivepolarity DPF somewhat better preionization is achieved with the“internal” antenna shown in FIG. 1 above.

FIG. 32 shows that the timing of the preionization pulse must beadjusted relative to the DPF main pulse to achieve optimum effect. Ifthe preionization is too early (as shown at 92) or too late (as shown at93) the efficiency of the deep plasma focus is adversely affected.

Preionize Injected Gas

Applicants have discovered that gases in metastable states are easier topreionize than stable gas. Gases can be put in metastable states byionizing them prior to injection into the discharge chamber. Forexample, FIGS. 2A(4), and 18A-E show gas injection techniques. In eachcase the injected gas could be placed in a metastable state by a highvoltage discharge (such as with 15 kV pulses with durations of a few ns)or by RF preionization. These metastable states last about 50milliseconds so with a gas flow of about 1 m/sec there will be plenty ofmetastable atoms if the ionizing discharge is about 5 cm upstream of theorigin of the pinch discharge.

Another technique useful when xenon is the active gas is to install anRF coil around the xenon inlet to the discharge region. Applicantspropose an RF frequency of 2 MHz to 2.5 MHz which causes a breakdown ofthe xenon gas in the inlet pipe. Alternatively, a high voltage pulseddischarge in the xenon inlet pipe could be used. In a preferredembodiment a magnetic field is applied to direct xenon ions so generatedto specific locations the pinch discharge is initiated.

Nozzle Assisted Preionization

The pressure for best production of EUV light in Applicants fourthgeneration devices is in the range of about 100 mTorr or less. Thispressure the discharge puts us on the left side of the Paschen breakdowncurve so that very high voltages are required for breakdown to produceionization. Ionization is much easier at higher pressures. A solution,consistent with the techniques described in the previous section, is toproduce the preionization in the nozzle used to inject either the bufferor active gas into the discharge chamber. Techniques for producing ionsin the inject pipe are discussed above. Another technique is to directionizing radiation to the injection nozzle from inside the chamber asshown in FIG. 31. This radiation is preferably discharge produced UVlight or x-radiation.

Hydrogen as Buffer Gas

Applicants have discovered that EUV optics in its prototype devicesbecome contaminated with carbon deposition. A 1 nm layer of carbon cancause a 1% relative reflective loss on multi-layer optics and more (upto about 10% for grazing incident optics). One known technique is to addoxygen to the buffer gas to react with the carbon to produce CO and CO₂.However, oxygen can also react with the optics producing oxide whichdegrades the optics.

Applicants propose to add hydrogen to the buffer gas preferably about20% to 50%. The hydrogen does not absorb at 13.5 nm, it etches carbonand it also reacts with oxygen. Also, the hydrogen could added onlyperiodically for short time periods as a part of maintenance program toclean the optics and removed after the optics are cleaned.

Optimization Techniques Optimizing Capacitance

Applicants have discovered that the highest plasma temperature existswhen the plasma pinch event occurs simultaneous with the peak of thecurrent flow from the drive capacitor bank. For a given anodeconfiguration and buffer gas density, the plasma front will travel downthe length of the anode in a given amount of time for a given amount ofcharge voltage. Maximum emission efficiency is obtained by adjusting thecapacitance value and charge voltage such that the peak capacitorcurrent exists during the plasma pinch event.

If a higher input energy level is desired and thus a higher chargevoltage, then the drive capacitance must be reduced so that the timingof the drive waveshape matches the plasma run down time along the lengthof the anode. Since energy stored on a capacitor scales as the square ofvoltage and linearly with capacitance, the stored energy will increaselinearly with voltage as one decreases capacitance proportional withincreases in voltage.

FIG. 13 is a drawing showing the measured drive capacitance voltage, themeasured anode voltage and the EUV intensity versus time for a preferredembodiment with the capacitance properly chosen to produce maximumcapacitor current during the pinch. In this case, for a 2 cm long anode,a He buffer gas pressure of 2.5 Torr and a C₁ capacitance of 3 μF.

Optimum Shape of Central Electrode

Applicants have discovered with hollow anode configurations, that theplasma pinch grows rapidly along the axis once the pinch has beenformed, and will extend down the opening in the hollow anode. As thispinch grows in length, it eventually drops too much voltage along itslength and an arc-over occurs across the surface of the anode. Onesolution to prevent this arc-over makes use of a blast shield to providea physical barrier to the growth of the pinch length extending away fromthe anode as described above. Another solution, to reduce the rate ofpinch length growth down into the hollow anode, is to increase the opendiameter inside the anode narrow region as shown in FIGS. 14C and14D(1). This slows the growth of the pinch length and prevent arc-over.All previous literature shows a hollow anode with a constant dimensionhollow portion. FIGS. 14A, 14B, 14C and 14D show examples of pinchshapes for various hollow anode shapes. The configuration shown in FIG.14D shows the shortest pinch shape.

Exposed Length of Central Electrode

Since the plasma run down time determines where on the drive voltagewaveshape the pinch occurs, Applicants have been able to adjust theduration of the pinch portion of the plasma focus device by changing theamount of exposed anode and thus the duration of the rundown. The buffergas density is dictated by a desired plasma pinch diameter, and thedrive capacitance is in practice limited to within a certain range.These two parameters, combined with the drive voltage determine thedesired run down time. The run down time can then be adjusted byincreasing or decreasing the length of exposed anode. Preferably, therun down time is chosen such that the plasma pinch event occurs duringthe peak in the drive current waveshape. If a longer plasma pinchduration is desired then the exposed length of the anode can be reduced,thus shortening the run down time and causing the plasma pinch to occurearlier in the drive waveshape.

RF Powered Vapor Production

Metal vapor delivery schemes described above depend on raising the anodetemperature sufficiently high that the vapor pressure of metal reached adesired level. Such temperatures are in the range of 1000-1300° C. forlithium and 2,260° C. for tin.

An alternative is to fabricate an RF antenna from a material such asporous Tungsten infiltrated with Lithium. This porous Lithium filledTungsten antenna 50 is placed down inside the anode as shown in FIG. 15.RF power source 52 creates a plasma-layer on and near the antenna willdrive off atoms that are swept up by the gas flow 54 through the centerof the hollow anode and the Lithium atoms carried to the end of theanode. The rate of metal ion production is easily controlled by thepower level of the RF source. In addition, the porous Tungsten anode canbe maintained with this RF drive at a temperature sufficient for liquidmetal to wick up from a reservoir 56 placed at the bottom of the anode.

Electrode Cooling Cooling of Central Electrode

In preferred embodiments of the present invention the central anode hasan outside diameter in the range of about 0.5 cm to 1.25 cm. The centralelectrode absorbs substantial energy due to the plasma fall duringdischarge and due to absorption of radiation from the plasma pinch.Cooling in the range of about 15 kw or more may be required. Because thegas pressure are very low there cannot be much cooling due to convectionthrough the buffer gas. Radiation cooling could only be effective atvery high anode temperatures. Conduction down the anode length wouldrequire a very large temperature drop.

Heat Pipe

If lithium vapor is used as an active gas and is injected through thecenter of the anode the anode temperature may need to be maintained attemperatures in the range of 1,000° C. to 1,300° C. or higher. This hightemperature of operation, substantial heat removal requirement, envelopeconsiderations and the high voltage limit the choices of coolingtechnique. One technology, however, a lithium (or other alkali metal)heat pipe, offers the potential for a relatively simple and robustsolution. Lithium heat pipes begin to operate efficiently attemperatures about 1000° C. The specific design of such devicestypically use refractory metals, molybdenum and tungsten, for the casingand internal wick and can therefore operate at very high temperatures.

The simplest embodiment would take the form of a tubular or annular heatpipe that is integral with the anode of the DPF for best thermalcoupling. A likely embodiment would be annular to enable the delivery ofliquid or vaporized lithium to the plasma of the DPF. By way of anexample, an 0.5 inch diameter solid heat pipe removing 15 kW would havea watt density of 75 kW/in² (11.8 kW/cm²). An annular heat pipe havingan OD of 1.0 inch and an ID of 0.5 inch removing 15 kW of heat wouldhave a watt density of 25.4 kW/in² (3.9 kW/cm²). Both of these examplesillustrate the potential of this technology since watt densities far inexcess of 15 kW/cm² have been demonstrated with lithium heat pipes. Inoperation, heat pipes have only a very small temperature gradient alongtheir length and can be considered as having constant temperature withlength for practical purposes. Therefore, the “cold” (condenser) end ofthe heat pipe will also be at some temperature at or above 1000° C. Toremove heat from the condenser end of the heat pipe a preferredembodiment may utilize radiative cooling to a liquid coolant (such aswater) jacket. Radiative heat transfer scales as the fourth power oftemperature, therefore, high rates of heat transfer will be possible atthe proposed operating temperatures. The heat pipe would be surroundedby an annular water heat exchanger capable of steady state operation at15 kW. Other embodiments may insulate the condenser end of the heat pipewith another material such as stainless steel and cool the outer surfaceof that material with a liquid coolant. Whatever technique is used, itis important that the heat pipe is not “shocked” with a coolant at thecondenser, i.e., forced to be much cooler than the evaporator end. Thiscan seriously impact performance. Also if the heat pipe temperaturefalls below the freezing temperature of the working fluid at any pointalong its length (˜180° C. for lithium) it will not work at all.

Restrictions to the operating temperature of components near the base ofthe central electrode (anode) may require that heat transferred to thisregion be minimized. This condition may be accomplished, for example, bycoating the exterior of the heat pipe with a low emissivity materialnear the region of lower temperature tolerance. A vacuum gap can then befabricated between the heat pipe and the desired lower temperaturecomponents. Since vacuum has very low thermal conductivity and the heatpipe is coated with a low emissivity material, minimal heat transferwill occur between the heat pipe and the cooler components. Maintaininga controlled anode temperature under varying power load levels isanother consideration. This may be accomplished by placing a cylinderbetween the heat pipe and the water cooled outer jacket. This cylinderwould be coated or finished for high reflectivity on its inner diameterand for low emissivity on its outer diameter. If the cylinder is fullyinserted between the radiating heat pipe and the water cooling jacket,radiation will be reflected back toward the heat pipe thus reducing thepower flow from heat pipe to jacket. As the “restrictor” cylinder isextracted a greater proportion of the heat pipe's condenser can radiatedirectly onto the water jacket heat exchanger. Adjustment of the“restrictor” position thus controls the power flow which sets the steadystate operating temperature of the heat pipe, and ultimately the anode.

A preferred embodiment using heat pipe cooling is shown in FIG. 16 shownin the drawing are anode 8A, cathode 8B, and insulator element 9. Inthis case, lithium vapor is used as the active gas and is delivered intothe discharge chamber through the center of anode 8A as shown at 440.Anode 8A is cooled with lithium heat pipe system 442 comprising lithiumheat pipe 444. Lithium within the heat transfer region 446 of heat pipe444 vaporizer near the hot end of the electrode 8A and the vapor flowstoward the cooler end of the heat pipe where heat is transferred fromthe heat pipe by radiative cooling to a heat sink unit 446 having a heatsink surface 448 cooled by water coil 450. The cooling of the lithiumvapor causes a change in its state to liquid and the liquid is wickedback to the hot end in accordance with well known heat pipe technology.In the embodiment a restrictor cylinder 452 slides up and down as shownat 454 inside heat sink surface 448 based on a drive which is part of atemperature feedback control unit not shown. The anode heat pipe unitalso preferably comprises an auxiliary heating system for maintainingthe lithium at temperatures in excess of its freezing point when theplasma pinch device is not producing sufficient heat.

Water Cooling of Central Electrode

Another preferred method of cooling the central electrode is shown inFIGS. 20, 20A, 21 and 22. In this case water under pressure iscirculated through the central electrode. Central electrode 8A as shownin FIG. 20C is comprised of two parts, a discharge portion 8A1 comprisedof single crystal tungsten (available from Mateck GMBH, Fuelich, Germanyand lower part 8A comprised of sintered tungsten. The outer electrode 8Bis made in two parts, a lid 8B1 and a base 8B2, both comprised of anoxide hardened copper material sold under the tradename Glidcop. Theoxide material is alumina. The outer electrode is made in two parts toprovide water passages 460 for cooling the outer electrode. Theelectrodes are insulated from each other by main insulator 462 comprisedof boron nitride or silicon carbide, a layer 464 of alumina deposited onstainless steel base 8A3 and a polymide 466 (preferably Kapton asavailable from Dupont). The water path through the central electrode isshown by arrows 468 in FIG. 20C. Cylindrically shaped stainless steelpartition 470 separate the supply and return flow in the electrodes.Parts 8A2, 8A2 and 8A3 are braised together using a gold/nickel orgold/copper braze material such as Niord or 50 An-50c.

Plasma Pinch with Radial Run-Down

Preferred embodiments of the present invention utilizes the pulse powerfeatures, the radiation collection features and the debris controlfeatures described above with any of the electrode arrangement asdescribed in FIGS. 2A, 2B, 2C and 2D. This electrode arrangementprovides advantages and disadvantages as compared to electrodeconfiguration such as that shown in FIG. 21. The electrodes have greatersurface area so that thermal problems may be minimized. There also couldbe less filamentations of the discharge and perhaps better plasmaconfinement and possibly better radial stability. Applicants believethey can design the electrodes to produce pinches along the axis of theelectrodes as shown in FIG. 21.

Use of Multiple EUV Sources

As indicated above a preferred application of the present invention infor lithography light sources for future machines, at least theproduction versions, have not yet been designed and built. It ispossible that illumination power may exceed the illumination power thatcan be conveniently produced by a single EUV source source utilizing thetechnology described herein. In this case two or more EUV sources couldbe combined to provide the illumination needed. Preferably the lightfrom each of the sources would be collected using techniques similar tothose described herein and projected on a single slit which would be thesource for the lithography equipment.

Integration with Litho Machine

In preferred embodiments portions of the EUV light source unit isintegrated directly into a lithography unit such as a stepper machine asshown in FIG. 2A(21). The integrated parts may include the commentatorand the compression head of the solid state pulse power unit and thevacuum vessel which includes the electrode set, debris shield andradiation collectors and turbo-molecular vacuum pumps all as shown at120 in FIG. 2A(21). Support equipment (including electronic controls,high voltage power supply, resonant charger, power distribution systemand fluid management for cooling water and gas control) are located in asupport equipment cabinet separate from the lithography unit (whichcould be in a separate room if desired) all as shown at 122. Roughvacuum pumps and high pressure water pumps are located in a thirdcabinet 124 which also could be in the separate room, in lithographyunit 126 are illumination optics, reticle, reduction optics and waferhandling equipment.

Electrode Erosion Minimizing Erosion

Applicants' experiments with their early prototype EUV device show thatelectrode erosion is a serious issue and Applicants have developedseveral techniques for dealing with this issue. Applicants havediscovered through experiments with their fourth generation plasma pinchdevice that the inductance in the discharge circuit increasesdramatically at the time the pinch occurs greatly reducing the currentflow. and producing an increasing electric field between the electrodes.As a consequence a second breakdown occurs between the anode and thecathode generally near the tip of the anode as shown in FIG. 2A(2). Thisproduces erosion at the location of the breakdown. Applicants propose tominimize this problem by providing a means for promoting their postpinch discharge at a location where erosion is not a problem. Onetechnique to inject a plasma containing gas in lower region between theelectrodes to produce the post pinch in this lower location far awayfrom the anode tip.

Sputter Replacement of Material Eroded from Anode

Applicants' experiments with its fourth generation device have shownsubstantial anode erosion with long-term operation. As indicated abovethe principal expected use of these plasma pinch devices is forintegrated circuit production. This means the device must operatesubstantially continuously for many days or weeks between maintenancedown times. Therefore, techniques must be found for increasing electrodelifetimes. A potential technique is to provide a sputter source forsputtering electrode material onto one or both of the electrodes. FIG.25 is a sketch showing two-tungsten sputter sources for providingsputtered tungsten to replace electrode erosion. Applicants discoveredthat short pulse high voltage driven electrodes used for preionizationwas producing sputter ions which collected on the sides of the anode andon the cathode. The side of the anode is also the location of most ofthe electrode erosion. Therefore, Applicants propose to providesacrificial electrodes of the same material as the anode and cathodespecifically designed to erode by sputtering. These sacrificialelectrodes will be positioned so that sputtered electrode material isdirected to regions of the anode and/or cathode suffering worse erosion.Preferably the sacrificial electrodes are designed so that they can beeasily replaced or periodically extended into the discharge chamber asthe erode. Some of the sputtered material will collect on insulatorsurfaces, but Applicants have leaned that sputtered tungsten depositedon insulator surfaces in these devices is not a problem.

Insulator Covered Electrodes

Applicants have discovered in actual experiments that center electrodeerosion can be greatly reduced by covering the side wall of the centerelectrode with insulator material. By covering with insulator materialportions of the electrode which would otherwise face high currentdensities, the post pinch discharge current is forced to spread out overa larger area in a different region of the electrode. This technique canbe employed to reduce the current density in the area of electron or ionimpact on the anode or cathode, respectively. The reduced erosion rateleads to reduced debris generation and longer electrode lifetime. Thereis still some erosion and debris from the sliding discharge across theinsulator, but it is not so severe as the electrode erosion. Theso-called “flash-over arcing” which leads to high erosion rates occursonly on conductive surfaces. It can therefore be eliminated in regionswhere the electrode is covered by the insulator.

Thus, a preferred embodiment is a dense plasma focus with the usualanode and cathode configuration, but without a sliding discharge alongthe outer diameter of the inner electrode (run-down length). Instead,the inner electrode is covered by a long insulator tube which protrudes,i.e., the diameter of the inner electrode is eliminated. Even thoughtthe effective inductance is slightly increased, an intense pinch stilloccurs on the axis leading to EUV generation. In contract toconventional dense plasma focus devices, there is no run-down occurringalong the inner electrode. The inner surface of the inner electrode mayalso be covered with insulator material to eliminate flash-over arcingin this region. This insulator has to have the appropriate innerdiameter in order not to reduce the pinch size and EUV output.

Preferred embodiments are in FIGS. 26A and 26B. In FIG. 26A insulator 60covers the outside surface and in the FIG. 26B embodiment insulator 62covers the inside surface in addition to insulator 60 on the outside.The anode in both FIGS. is identified at 64 and the cathode at 65.

Pyrolytic Graphite Electrodes

In a preferred embodiment the discharge surface of the anode shown at 8Ain FIG. 2A(2) is covered with pyrolytic graphite. The body of the anodeis copper or tungsten. An important advantage of this design is thatcarbon is 15 times lighter than tungsten (the principal prior art anodematerial). Therefore, the carbon debris is much easier to deal with in adebris shield. Also graphite does not melt; it evaporates. Preferablythe graphite should be applied so that the atomic graphite layers arealigned perpendicular to the surface to improve thermal conductivity andto minimize erosion. Preferably an interlayer is applied between thepyrolytic graphite surface material and the substrate electrode materialto minimize thermal stresses.

Electrode Replacement Shutter With Seal

When the plasma focus source components and collector are contained inthe same chamber any maintenance of the source requiring venting willhave disadvantageous effects on the collector mirrors and also on thedebris trap. A separation of these components into two chambers withrespect to vacuum should be very beneficial. However, prior art designswith respect to position of debris trap and collection optics just donot provide the space required to accommodate a gate valve between thetwo chambers.

Applicants have developed techniques for venting the source chamber formaintenance (like electrode replacement) while keeping the collectorchamber under (near-) vacuum during this time. The source chamber 69will require more frequent venting compared to the venting required forthe collector chamber 70. The collector mirrors 66 and also the debristrap 68 will be protected when maintenance is carried out on the sourceby use of the proposed shutter. Therefore the lifetime of the collector(and perhaps also of the debris trap) will be greatly increased. Since avery short distance is required between the pinch source volume 71 andthe debris trap and collection optics entrance in present designs, thereis usually not enough space available to accommodate a separating gatevalve. When the proposed shutter with seal towards the collector chamberis introduced, only very little space is required to accommodate it. Thecollector chamber can be kept under (near) vacuum, since the shutterwill be pressed against the sealing surface by the ambient pressure ofthe vented source chamber.

The advantage of the present design is illustrated in FIGS. 27A and 27B.The prior art drawing FIG. 27 shows an arrangement with a gate valve 72separating the source and collector chambers. However, present designsrequire a distance of 100 mm or less from the plasma source volume tothe entrance of the grazing incidence collector optics and thus usuallydo not provide enough room to accommodate a gate valve. UHV gate valvesfrom vacuum suppliers like VAT with 8 inch (200 mm) or 10 inch (250 mm)opening diameter have a flange-to-flange distances of 80 to 100 mm.Therefore, such gate valves are omitted in present designs. This has thebig disadvantage that each time when venting for maintenance of thesource is required, the collector chamber is also vented. Each ventingcycle has disadvantageous effects for the very sensitive collectoroptics. Furthermore, the pump-down time is longer for the collectorchamber compared to the source chamber since its vacuum requirements aremore severe. If the collector chamber does not need to be vented eachtime when the source chamber is vented, several advantages exist: Thecollector optics contamination is reduced and the optics lifetime isincreased. The system maintenance down-time is decreased because nopump-down of the collector chamber is required at the end of themaintenance work. The sensitive debris trap is also protected better.

FIG. 27B shows a proposed mechanical shutter 74 with vacuum seal fromthe source to the collector chamber. The shutter has an o-ring seal onthe side facing the collector chamber just like the plate of a gatevalve. The space required to accommodate this shutter is only 20 mm orperhaps even only 10 mm. In contrast to a gate valve the shutter canprovide a vacuum seal only with respect to collector chamber and not forthe source chamber. However, this is sufficient, since in most casesonly the source chamber needs to be vented (shutter in closed positionas shown in the figure). When the collector chamber needs to be vented,the source chamber can always be vented, as well, without anydisadvantages (shutter in open position).

When the shutter is approaching the closed position, it is pressed withits o-ring seal against the sealing surface of the collector chamber bya notch or protrusion near the shutter end position. The sealing surfacemay be conveniently located on the outer circumference of the debristrap (holder), for instance. At the start of the source chamber venting,the increased pressure in the source chamber will push the shutterfurther against its sealing surface with a force which will increasewith the increase of the pressure in the source chamber. At thebeginning of the venting some small leaks may still exist towards thecollector chamber, but this can be tolerated. When the source chamber isat high (atmospheric) pressure, the force pushing the shutter againstits sealing surface will be so large due to the relatively large shutterarea that a high-vacuum seal is established. This is sufficient toprotect the collector optics (and debris trap). A (minor) disadvantageis that the sealing shutter has to be integrated into the collector (orsource) chamber design (preferentially right next to the connectingvacuum flange). But the major advantage is that the space required forthe extra 2 flanges of the gate valve and some of its width can beavoided. Therefore, such a shutter can be accommodated even when therequired separation from the source to the debris trap/collectorentrance is very small.

Replaceable Electrode Module

Another technique to simplify electrode replacement is to design the EUVdevice for replacement of the electrode, the debris collector and thefirst collector as a single module. For example, referring to FIG. 19,collector 42 would a port of a module comprised of anode, cathode anddebris collector and collector 42. The system would permit thesecomponents to be replaced as a unit in a minimum period of time toreduce maintenance down-time. This results in quick replacement of theelectrodes which degrade because of erosion and the debris collector andfirst collector optics which degrade because of contamination witheroded material.

Example of an Optimized Dense Plasma Focus Device Optimization Efforts

Applicants have devoted considerable effort to optimize performance oftheir fourth generation dense plasma focus device shown in cross sectionin FIG. 2A(1) for efficient generation of EUV radiation. A side view ofthe system with vacuum chamber is shown in FIG. 2A(3). Performanceparameters included in their investigations are He and Xe pressure andflow rates, electrode geometries, pre-ionization characteristics, andduty factor related performance issues. In these investigationsApplicants found that the location of the He (buffer gas) and Xe(working gas) gas injection ports as well as the pressures and flowrates of the gas mixture components had a strong impact on EUV emissionefficiency. Additional constraints on the gas recipe are also derivedfrom gas absorption of the EUV radiation and the desire to providedebris mitigation properties. Best results to date have been obtainedwith an axially symmetric buffer gas injection scheme coupled with axialXe injection through the central electrode. The highest conversionefficiency obtained was 0.42% at 12.4 J of input energy. Measurements ofenergy stability show a 10% standard deviation at near optimum EUVoutput. The matching of the drive circuit to the pinch as determined bythe damping of the voltage overshoot waveforms was found to dependstrongly on the He and Xe pressures. Energy Dispersive X-Ray (EDX)analysis of the debris emitted from the source shows that the primarysources of the debris are the central electrode and the insulator. Noevidence of cathode material has been found. In addition to effortstoward more efficient operation, first phase efforts of thermalengineering have been undertaken, which have led to continuous operationat 200 Hertz with conventional direct water-cooling. The system can beoperated at higher repetition rates with proportionally lower dutycycles. The data shows the distribution of thermal power throughout thewhole system. This more detailed understanding of the thermal power flowallows Applicants to better determine the ultimate high volumemanufacturing potential of this source technology.

Applicants have demonstrated significant gains in performance withconversion efficiencies approaching those of the more mature laserproduced plasma sources. The particular specifications which the lightsources must meet are tightly coupled with the design of the entireillumination system. Key source parameters which must be measured are:operating wavelength, in-band EUV power, out-of-band power, source size;maximum collectible angle, high repetition rate scaling; pulse to pulserepeatability and debris generation from plasma facing components.

Applicants' early efforts in DPF development were directed at developingthe basic pulsed power technology required to drive a source of thistype. High conversion efficiency was demonstrated with Li vapor as theactive radiating element at high stored energy (25 J). These storedenergies were too high for practical scaling to high repetition rateoperation. Development of the 4th generation machine allowed Applicantsto use Xe as the active species. Their recent efforts have been focusedon optimizing the performance of the DPF with Xe as the source gas. Tofacilitate this effort they have investigated pulsed power development,plasma initiation and characterization, EUV metrology, debris mitigationand characterization, thermal engineering, and collector opticsdevelopment.

System Description

The fourth generation of Dense Plasma Focus system developed byApplicants utilizes a power system with solid-state switching andseveral stages of magnetic pulse compression (as shown in FIG. 1 anddescribed above) similar to that used in Cymer's excimer lasers, inorder to generate the high voltage, high peak power pulse required bythe DPF to generate EUV light. These systems begin with a chargingvoltage of 1300 V and generate an output pulse applied to the DPF of ˜4kV with a risetime of less than 50 ns. Although current measurementshave not yet been directly performed, circuit simulations based on thevoltage waveforms from typical experiment operation predict that theoutput DPF drive current peaks at a value of ˜50 kA, with a dI/dt of 675kA/∝s. It is this combination of high peak current and high dI/dt thatallow the DPF to function efficiently.

The most important features of this fourth generation device isdescribed in FIG. 33 along with a bullet list of the advantages of thedeep plasma focus device. As explained elsewhere Applicants havedemonstrated conversion efficiencies (the ration of: in-band EUVradiation at an intermediate focus to electric power input) of about0.5%. As of the filing of this application, Applicants have demonstratedthe following system performance parameters:

Current Source Performance EUV efficiency with Xe, (2% BW, 2π sr) >0.45%EUV energy per pulse (2% BW, 2π sr) ~55 mJ Average source size (FWHM)~0.4 × 2.5 mm Source position stability (entroid) <0.05 mm, rmsContinuous repetition rate 1000 Hz Burst repetition rate 4000 Hz EnergyStability ~7%, rms Avg. EUV Output Power (2% BW, 2π sr) 50 Watt EUVoutput Power, Burst (2% BW, 2π sr) 200 Watt

Collection efficiency is about 20 to 30 percent and about half of thecollected EUV in band radiation can be delivered to the intermediatefocus utilizing the technology described herein. Thus, the demonstratedEUV power at the intermediate focus is currently about 5 Watts on acontinuous basis and 200 Watts in burst mode. With the improvementsdescribed herein Applicants expect to increase the continuous power atthe intermediate focus to at least 45.4 Watts within the near future andultimately to 105.8 Watts. Burst mode performance will be roughlyproportionately greater.

Six fourth generation DPF machines have been built and are being usedfor a variety of experiments on system optimization, pre-ionization,power system development, debris mitigation, thermal management, andcollector design. For those experiments not requiring high repetitionrates (˜1 kHz and above), charging power for these machines is simplyprovided by resistive charging from a set of DC power supplies. ThoseDPF systems that do require high rep-rate capability are being chargedwith a resonant charging system which charges the initial energy storagecapacitor, C0, to a voltage of 1300 V in less than 250 μs. Theseresonant charging systems also provide energy recovery, storing theenergy which is not utilized by the DPF or dissipated in heat and usingthis recovered energy for the next pulse. This reduces the amount ofpower required by the main power supply and also helps with other issuessuch as thermal management.

Measurements

In this section Applicants present an overview of measurements performedon one of Applicants low-duty-factor sources operated at less than 50Hz. They show the dependence of the EUV output and conversion efficiencyon gas recipe, present data on the out of band emission, and showmeasurements of the source size and position stability.

Over the past year significant progress has been made in understandingsome of the empirical dependencies of the EUV output on electrodegeometry and gas dynamics issues. Significant changes in the apparatus,as compared with earlier generations include a new cathode design whichallows gas to be injected symmetrically around the anode region, and asystem for injecting He and Xe mixtures through the anode electrode. Thegas delivery system was modified to allow combinations of He and Xe tobe injected into different sections of the DPF system. A schematic ofthis system is shown in FIG. 1. Gas control is performed via two massflow controllers and a high accuracy capacitance manometer. The systemis operated in a constant-pressure mode. Xe is injected in aconstant-flow mode, and He makeup gas is added in order to reach thetarget operating pressure. In this mode the He flow rate depends on thepumping speed of the system. Dependence on gas flow rates wasinvestigated by testing different pumping configurations.

Radiation emitted from the pinch along the axis passes through anaperture into a differentially pumped diagnostic chamber 204. Gasabsorption in the measurement vessel is minimized by maintaining thepressure below 5 mTorr. For these measurements the diagnostic vesselentrance was located 5 cm from the pinch region. No correction for thegas attenuation in the main DPF vessel along the 5 cm path nor in thediagnostic vessel is performed. The radiation from the pinch isreflected from a Mo/Si multi-layer mirror and is directed through a 1 ∝mthick Be foil onto an un-coated IRD AXUV-100 photodiode. A typicalmeasurement sequence consists of recording the voltage waveforms on thepulsed power system, the DPF anode, and the photodiode as a function ofthe experimental parameters. Data acquisition and control of the gassystem are performed via a computer interface.

The representative dependence of the in-band EUV signal (at 13.5 nm,into 2% bandwidth, into 2π sr) on the Xe flow rate is shown in FIG.2A(5) at a constant operating pressure of 350 mTorr and at a fixedcharging voltage on the first stage capacitor of the pulsed powersystem.

A significant increase in the EUV output from the source was observedwhen He was injected around the anode and Xe through the cathode at 20Hz source operation compared with He injection into the main DPF vessel.Additional improvement was observed by increasing the He gas flow ratevia the addition of pumping capacity. The effect of higher pumping speedis to make the EUV output less sensitive to the Xe mass flow set pointand to increase the measured EUV output.

Similar measurements were performed as a function of He pressure at aconstant Xe flow rate and a voltage of 1300 V on the first capacitorstage C2 as shown in FIG. 1. FIG. 2A(6) shows the voltage waveform onthe final stage capacitor (C2) and the in-band 13.5 nm photodiode signalfor He injection around the anode. The EUV signal strongly depends onthe He pressure. Examination of the C2 waveform shows that the energyrecovered by this capacitor due to underdamped response depends on thegas recipe. A similar dependence was observed as a function of Xe flowvariation.

The energy dissipated in the pinch region is calculated from thedifference in stored energy on the C2 capacitor. At 1500 mT He pressure,approximately 70% of stored energy is dissipated in the pinch region(8.8 J), while at 200 mT, the corresponding value is 96% (11.9 J). Thisdependence is illustrated in FIG. 2A(7) where the photodiode signal,initially stored energy, recovered energy and dissipated energy areplotted as a function of the He pressure. The EUV signal increases byapproximately a factor of 10 over this range. A further decrease in theHe partial pressure results in a sharp drop in the EUV yield not shownin these data.

Another interesting feature of the dependence on the gas pressure is theshift in the onset of EUV emission as measured by the photodiode. At theconstant Xe flow conditions used, this variation of the pressure from180 mT to 1500 mT results in a shift of 150 ns of the EUV emission. Fromclassical snowplow and slug models of the DPF operation Applicantsexpect the characteristic axial and radial transit times for the plasmashock front to scale with the square root of effective mass density.This scaling needs to be confirmed for this configuration, and theproportionality constant may be related to the effectiveness of theshock front in sweeping the mass out of the electrode region.Calculations of this effect, based on a one-dimensional snowplow modelsuggest that axial and radial effective masses may be significantly lessthan those derived from the actual gas pressure.

The dependence of the average in band EUV energy and energy efficiencyon the dissipated energy at fixed gas flow conditions is shown in FIGS.2A(8) and 2A(9), and 4 b. These data were taken with the sourceconditions optimized at the peak EUV output. Lower energy input wasobtained by reducing the charging voltage while leaving all otherparameters fixed. The data shown here are for the optimum conditions ofthe present experiment as well as for the configuration presented in [1]employing a different gas recipe and anode geometry. At 10 J a 70%increase in conversion efficiency (CE) is obtained, as compared with theprevious configuration. Although the energy coupled into the pinchdepends on the gas recipe, we can see that the dependence of EUV energyshown in FIGS. 2A(6) and 2A(7) is primarily due to variation in gas flowand not to the change in coupling.

Two types of measurements of the EUV radiation lying outside the 2%bandwidth around 13.5 nm were performed. The experimental setup forthese measurements is shown in FIGS. 2A(8) and 2A(9). The first type ofmeasurement compared the total radiation from the pinch on axis with thefraction transmitted through a CaF2 window transmitting in the 130nm-1300 nm band. These results show that ˜0.5% of the total radiationemitted from the pinch lies in the CaF2 band between 130 nm and 1300 nm,and are similar to previous results obtained by Applicants. In thesecond measurement the fraction of radiation emitted from the pinchreflected from one Mo/Si multilayer (ML) mirror and detected by theAXUV-100 photodiode was compared with the fraction of radiationtransmitted through a 1 ∝m Be foil and reflected by the ML mirror. Thesignal, measured on the photodiode with no filter in place afterreflection from the ML mirror gives the sum of in-band and out-of-bandcomponents. Insertion of a Be filter limits the measurement to thein-band fraction only. Therefore by subtracting the in-band fraction ofradiation corrected for the ML mirror transmission from the total signalwith no filter we conclude that ˜15% of the total radiation reflected byone ML mirror is out of the 2% band around 13.5 nm.

Measurements of source size and centroid motion were performed with thesource tuned for peak output. A pinhole camera employing aback-illuminated CCD array and a Be filter was used. The source imagesare shown in FIG. 2A(10). These images were taken with the camerapositioned on axis. Measurements were also taken at an angle of 68degrees. The average source size (averaged over 100 pulses) wasdetermined to be 0.25 mm×2 mm full-width-at-half-maximum. Thepulse-to-pulse EUV source centroid displacement is plotted in FIG.2(A)11. The average displacement is approximately 50 ∝m.

Using the pinhole camera technique we can obtain an estimate of the EUVenergy stability by integrating the intensity in each frame andcalculating the standard deviation of this quantity. The results show9.5% (1

) intensity fluctuation. This measurement compares well withmeasurements of the energy stability performed with the standardmeasurements using a Be filter, ML mirror, and AXUV-100 photodiode.Additional experiments that will be performed with this diagnostic willinclude correlation of EV source size with the in-band energy.

From the on-axis images we also conclude that there is no EUV productionoriginated from an interaction of the pinch with the anode end wall. Themaximum EUV intensity is observed in the center of the pinch where Xegas is injected through an aperture in the anode. No EUV emission isobserved at the periphery of the pinch where it contacts the anode endwall.

High Repetition Operation

Stable operation of the source at high repetition rates is important forhigh exposure dose and accurate dose control. The burst mode operationof this fourth generation light source was improved. Using a resonantcharging scheme with 10 J input energy (similar to that employed byCymer's excimer lasers), the maximum burst emission period was increasedto up to 300 pulses at repetition rates of 2 KHz.

The time-integrated in-band energy of the EUV pulses was measured usingthe multi-layer mirror—Be foil—photodiode detection scheme describedabove. The in-band energy vs. pulse number data are shown in FIG.2A(12). When the repetition rate was increased from low to high rateswith no changes of the gas mixture, a severe reduction of the EUV outputenergy was observed with increasing burst pulse number. By makingappropriate adjustments of the gas recipe it was possible to tune theoutput in order to obtain relatively stable EUV pulse energy for 300pulses long bursts at a 2 kHz repetition rate. As shown in the figure,after a transient period lasting for about 10-15 pulses the outputenergy stays at high values for the remainder of the burst. Thecorresponding measured standard deviation of the energy stability inthis mode is 10%. At the present level, we have not reached anyfundamental scaling limitations for high-repetition-rate operation and afurther performance increase should be possible with upgraded pulsedpower and thermal management schemes.

Debris Mitigation

Applicants have exposed Mo and Pd coated silicon wafers to the debrisproduced by the DPF in an effort to evaluate the main source of thedebris, and the debris deposition rate on the collector optics. Thesource configuration for these tests consisted of a tungsten anode,alumina insulator and brass cathode. Samples were exposed to 4.105pulses at 30 Hz, at a distance of 5 cm (Mo sample) and 11 cm (Pd sample)away from the pinch. The arrangement and placement dimensions are shownin FIG. 10. After exposure the samples were analyzed by EnergyDispersive X-Ray (EDX) analysis. The results, summarized in Table 1below, show that anode (W) and insulator (O, Al) materials were found atboth distances, 5 cm and 11 cm.

No sign of cathode material was observed. A small fraction of Xe wasfound on the Mo sample at 5 cm. This may be a signature of energetic Xeions produced by the DPF or simply Xe incorporated into the thin filmcoating. The presence of He could not be detected by EDX. The presenceof a weak but detectable Mo signal at 5 cm is an indication that thedeposited debris is between 0.5 ∝m and 2.0 ∝m thick, which is thetypical penetration depth for EDX analysis.- This gives us an estimateof the debris generation rate at 1-4.10-3 nm per pulse on axis at 5 cmfrom the pinch.

A simple optical technique was tested to characterize the deposition ofdebris generated by the DPF. The absorption of metals in the visibleregion of the spectrum is generally high. The corresponding opticalthickness up to which appreciable transmittance occurs is generally wellbelow a quarter wavelength in this region so that interference fringesare not observed. According to Lambert-Beer's law:T=e ^(−α*L)where T is the transmittance, α is the absorption coefficient and L isthe film thickness. Therefore the absorbance A, defined as Log₁₀(1/T),is proportional to the film thickness if α is independent of L. If L isproportional to the number of pulses, then from a measurement of theabsorbance of a coating on a transparent sample due to debris producedby the DPF as a function of the number of pulses the debris depositionrate per pulse may be determined. Experimental verification of thisproportionality is plotted in FIG. 11.

Measurements of the absorbance allow one to compare the debrisdeposition rate on witness samples under different DPF operatingconditions. We used this method as the primary means for obtaining theangular distribution of the debris, as well as for the debris reductionfactor due to the insertion of a debris shield.

To evaluate the effectiveness of the debris shield concept a simplesingle-channel test setup was designed and built. The geometry andcritical dimensions are shown in FIG. 2A(15). Glass samples were placedat 6 cm from the pinch either facing the pinch directly or after aseries of metal cylinders with 1 mm diameter channels drilled throughthem. Tests were performed with 1 cm and 2 cm channel lengths. Duringthe tests total pressure in the chamber was 0.7 Torr with Heliuminjection into the main vessel and Xe was injected through the anode. Bycomparing the debris film thickness using the absorbance technique, forsamples which were exposed to the same number of pulses at the sameoperating conditions but with different debris shield lengths, we cancalculate a debris reduction factor (F). If F=1 is defined as the casewhen the sample was placed without any protection, then F shows howeffectively the debris shield protection works. Experimental results forthe 1 and 2 cm thick single channel setup are plotted in FIG. 2A(17).These results show a reduction factor of 100 per cm of shield length.These results may be compared with the reduction factor measured for amore realistic multi-channel debris shield shown in FIG. 2A(16). Thisprototype shield was fabricated from stainless steel by electrondischarge machining (EDM). The data show that under these conditions thereduction factor measured for the 1 cm long multi-channel shield wascomparable to the simple 1 cm single channel setup. This gives us ameasure of confidence in scaling this type of debris shield to thelength required for practical source operation.

Thermal Engineering

Water-cooled electrodes, the first step in development of a thermalmanagement solution for the DPF discharge region, have been designed andtested on Applicants fourth generation EUV light source. Theseelectrodes have enabled study of the DPF operation at significantlyhigher steady-state repetition rates than previously achieved andgenerated calorimetric data that shows the dissipation of thermal energyin each electrode.

The cathode has four separate cooling delivery and exhaust loops, onefor each quadrant of the annular weldment. The flow through eachquadrant is arranged to be similar. It was designed to maximize the areacooled internally by the water and minimize the conduction path throughthe plasma heated wall and was fabricated from a high thermalconductivity copper alloy with good mechanical properties. At 400 kPathe total water flow through the cathode is 3.8 liters per minute. Thewater-cooled electrodes are shown diagrammatically in FIG. 2A(18). Theanode is cooled by flowing water through two concentric, annularchannels created in the body of its welded assembly. This allows thewater to get very close to the region of the part heated mostaggressively by the plasma. Water can be pumped through this electrodeat relatively high pressures giving high water flow rates andmaintaining a more favorable temperature gradient in the region ofhighest heat flux. In recent testing water has been pumped through theanode at 1100 kPa giving a flow rate of 11 liters per minute.

Testing of the water-cooled electrodes has been carried out up toseveral hundred Hertz in short bursts and at steady state repetitionrates up to 200 Hz. The results so far indicate that a reasonablecorrelation exists between measured electrical energy input and measuredheat load on the electrode cooling system when other as yet unmeasuredbut largely understood system heat losses are considered. The thermalenergy leaving the electrodes in the water is not divided evenly betweenthe anode and cathode. Typically the cathode removes more heat than theanode. The data suggest that the cathode removes a higher proportion ofthe heat as the repetition rate rises. This was expected since the anodetemperature rises more rapidly than that of the cathode with increasingrepetition rate and the corresponding reduction of thermal conductivityin the anode material is significant. The cathode also has a much largercooled area, a shorter heat conduction path and far higher thermalconductivity than the anode. The fraction of heat removed by eachelectrode is shown in FIG. 2A(19).

A summary of the demonstrated source parameters is given in FIG. A(20).In the past year Applicants have built five new DPF sources as well asimplementing upgrades to our existing fourth generation system bringingthe total number of operational systems at Cymer to six. Significantimprovements were made in the conversion efficiency primarily byoptimization of the gas recipe and gas injection geometry. The bestachieved conversion efficiency into 2π sr and 2% bandwidth was ˜0.4% at˜10.5 J and low repetition rate. Stable EUV output was demonstrated for300 pulse bursts at 2 kHz using our proven resonant charger technology.Experiments performed to date suggest that further improvement ispossible by continued optimization of the gas delivery system. Energystability continues to be ˜10% (1σ) and will require improvement. Out ofband radiation is <0.5% for the improved CE source.

Characterization of debris collected on witness samples exposed to thepinch shows deposition primarily of anode material (W), and anodeinsulator material (Al, O). No evidence of cathode material is seen.Measurements of the debris reduction factor for single and multiplechannel debris shield show a reduction factor of 100× per cm of shieldlength. Extrapolating this result to a reduction factor of 10 8 suggeststhat a 4-5 cm shield will be required.

The measurements of heat extraction from the electrodes for continuousoperation at 200 Hz show that approximately 60% of the power isdissipated in the cathode with 40% going to the anode. This suggeststhat at 5000 Hz repetition rate and 10 J total input energy we wouldneed to extract approximately 20 kW from the anode electrode. At theseconditions using 0.4% CE we calculate a total in band radiated power of200 W into 2% BW and 2π sr at the source. Appropriate reduction factorsmust be used for all downstream components that attenuate the sourceradiation.

Other Improvements Dual Purpose Collectors

Due to large reflection losses of EUV mirrors, minimization of thenumber of mirrors is very desirable for illumination systems for EUVlithography. Specially designed surfaces can have additional featuressuch as beam homogenization features. One such feature could be areflective diffuser added to a grazing incidence collector of the typedescribed above.

Use of Magnetic Field and Preionizers to Control Pinch

Applicants have demonstrated that magnetic fields can be used to controlthe pinch size and position. In one embodiment a permanent magneticpositioned above the pinch region reduces the pinch length. Magnets canalso be positioned in the aode as shown in FIG. 28A. Magnetic fields canalso be applied to help confine the pinch. Applicants have alsodemonstrated that the shape and position of the pinch can also becontrolled by moderating the preionization signal from preionizers 138as shown in FIG. 2A(2).

Metal in Solution Target

Metals such as lithium and tin provide vapors which make good activegases to produce radiation in the 13.5 nm range. However, dealing withmetal vapors is difficult. A technique for providing target material atthe pinch site is to form a liquid solution with the metal and injectthe target in liquid form.

When a liquid solution containing the metal is inserted into thedischarge chamber, the metal does not have to be heated for delivery.The target delivery can be made in a so-called mass-limited way, i.e.,just the right amount of metal (particles) is delivered, no more massthan needed. This leaves no extra particles, which would otherwise justrepresent unwanted debris produced by the source. The target materialcan be delivered in a liquid jet from a nozzle, if a sufficiently highbacking pressure is applied. In this way, it can be delivered to thedischarge region and it can be avoided that the whole discharge chamberis filled with target material. Since colloidal particles in suspensionor liquids or particles in liquids are used, the target density can bemuch higher than for metal vapor. By choosing the right concentration ofmetal content of the liquids, an optimized mass-limited metal target canbe provided. It is also much simpler to just inject a liquid into thechamber rather than constructing a metal vapor delivery system, forinstance based on a heat pipe principle. Tin nitrate should be anefficient target for 13.5 nm to 14 nm EUV light generation.

An improvement in EUV output and preionization was observed when apulsed magnetic field was applied by means of a coil mounted as shown inFIG. 28B below. The coil current pulse is shown in FIG. 30. This pulseproduces a magnetic field between 200 and 500 G at the end of the anode.An improvement in preionization was seen as shown by the anode waveformin FIG. 29A. The corresponding change in C2 waveform is shown in FIG.29B. The application of the pulsed field resulted in a higherpreionization density in the anode cathode region as evidenced by thedrop in anode voltage shown in FIG. 29A. The EUV output increased withthe pulsed field. The in band EUV waveshape is shown in FIG. 29C with Bon and off. The overall dependence of the EUV output on input energywith pulsed field applied is shown in the upper curve of FIG. 29C. Thecurves below this are with no pulsed B field. FIG. 2A(9) showsimprovements in efficiency resulting from electrode geometry improvementdiscussed herein including gas pumping and preionization changes andplasma dynamics using magnetic effects.

Metal targets can be delivered by means of liquids, fluids, solutions orsuspensions. The compound has to be liquid at the given (backing)pressure at temperatures around room temperature, say, from ˜10° C. to˜50° C. This technique applies to any pinched (=magneticallyself-compressed) discharge which can produce EUV or X-ray radiation,like a dense plasma focus (DPF), a Z-pinch, an HCT-pinch (=hollowcathode triggered pinch) or a capillary discharge. The liquid can bedelivered through the former gas injection port of the discharge device,see FIG. 18A for example for the case, when the discharge device is aDPF. In another embodiment see FIG. 23, the liquid can be at highpressure or can be backed up by very high-pressure (ca. 80 atm) heliumgas and be delivered to the discharge region via a jet nozzle with verysmall opening (ca. 50 μm to ca. 10 μm). In this way, themetal-containing liquid is confined to a narrow liquid jet. The jetcrosses the pinch region of the discharge. Additional gas may beinserted to promote the development of an efficient pinch discharge. Theliquid and evaporated gas can be pumped away by a nearby dump port witha vacuum pump. The nozzle expansion through the nozzle or through theinner electrode may also alternatively be operated such as to form aseries of liquid drops or as a (more diffuse) liquid spray expansion.The liquids provide an easy means of delivering metals of optimalconcentration, diluted in solution, to the discharge region. Heating ofthe metal to provide a metal vapor can be avoided.

The preferred metals are the ones that provide efficient EUV generationin the region of ca. 13 nm to ca. 15 nm. They are: lithium, tin, indium,cadmium and silver. Lithium (Li2+) has a strong transition at 13.5 nm.Tin (Sn), indium (In), cadmium (Cd) and silver (Ag) have strong 4d-4ftransition arrays from several ion species overlapping in the 13 to 15nm wavelength region. (As one goes from 13 nm to 15 nm, the peakreflectivity of the multi-layer mirrors for EUV lithography decreases,but their bandwidth increases at the same time. Therefore, the integralreflected intensity can still be large, and wavelengths above 14 nm arestill of interest here.) The preferred solutions are alcohols likeiso-propanol, methanol, ethanol, etc., and also water or glycol.

The preferred chemical compounds are lithium fluoride, lithium chloride,lithium bromide-salts, dissolved in water, for instance. For Sn, In, Cdand Ag preferred solutions are likewise chlorine solutions, brominesolutions and fluorine compounds. In addition, metal sulfates andnitrates.

Tin nitrate (Sn(NO3)4) is one of the most interesting compounds.Likewise, indium nitrate (In(NO3)3), cadmium nitrate (Cd(NO3)2), andsliver nitrate (Ag(NO3)). Nano- and micro-particles in solution orsuspension may also be used. It may also be considered to insert suchnano- and micro-particles by turbulence into a gaseous stream of heliumand not use a liquid at all for delivery.

Additional EUV Light from Electron Impact

Applicants propose to supplement the in-band light produced by itsplasma pinches with light results from energetic electron impact.

Bremsstrahlung (=soft x-ray radiation) generated from energetic electronimpact on solids with suitable absorption edges generates EUV radiationin addition to the EUV radiation produced in the gaseous pinch plasma.This is the idea in general. In the case of our DPF source, forinstance, it is known that when operated with positive polarity on thecentral electrode (=anode), an electron beam (with electron energies ofseveral keV is produced which impinges onto the inner front side of thecenter electrode. For 13.5 nm radiation, Si (silicon) is the suitablematerial to be placed here. The silicon L-absorption edge occurs at 13.5nm. Therefore, the energetic electrons will produce 13.5 nm radiation.This is completely in addition to the main 13.5 nm-radiation produced bythe xenon ions in the pinch plasma. Therefore, more EUV radiation willbe generated, if the central inner potion of the anode (in general, anyplace where the electron beam impacts) is made out of silicon. Anelectron kinetic energy of 10 keV is just about right for optimalefficiency. For instance, put silicon inside of the tungsten anode.Without silicon oat the place f impact (=present mode of operation),there is no match of the absorption edge (e.g., tungsten), consequentlyno additional radiation is produced at 13.5 nm. Silicon is of mostimportance here, but the principle applies also to other materials atother wavelengths. (For example: Beryllium insert to produce 11.5 nmradiation at the Be K edge). A sketch showing this technique is providedin FIG. 24.

Metal Vapor Produced by Sputtering

In preferred embodiments the active gas (lithium or tin vapor) andpre-ionization is provided in a single system. In this case the metaltarget is sputtered with an electric discharge which produces the metalvapor and also produces any ionization needed to promote the maindischarge. The source for the sputter power preferably is a signalgenerator, a 100 Watt linear RF amplifier and a 2000 Watt commandamplifier. The solid lithium or tin target is preferably located in ahollow in the central electrode and the sputter discharge are directedto that target.

For example, Applicants fourth generation EUV sources produce about 5Watts of in band EUV energy at the interim focus 11 in FIG. 19.Applicants expect future design using existing technology to boost this5 Watts to about 45.4 Watts. However, some designers of EUV lithographymediums have expressed a desire for power levels of more than 100 Watts.Applicants propose to accomplish this by combining two EUV sources usingthe technology described herein into one EUV system.

Wavelength Ranges

The various embodiments discussed herein have been discussedparticularly in terms of light sources producing ultraviolet in thespectral range of between 12 and 14 nm. This is because mirror suppliershave reported substantial success in the development of multi-layer nearnormal mirrors for UV light within these wavelengths ranges. Typicallythese mirrors have maximum reflectivities of about 0.6 to 0.7 in the 12to 14 nm range and the mirrors typically have a FWHM bandwidth of about0.6 nm depending on the specific mirror design. So the typical mirroronly covers a portion of the spectral range between 12 nm and 14 nm.

For this reason it is very important to carefully match the spectraloutput of the source to the spectral range of the reflectivity of themirrors which will be used to direct the beam, such as the mirrors inlithography scanner machine.

The reader should also understand that the teachings of thisspecification will apply to a much broader spectral range than the 12 nmto 14 nm range where most of the current extreme UV attention isfocused. For example, good mirrors can be produced for the 11 nm rangeand there may be advantageous for using these pinch devices atwavelengths above the 14 nm range up to about 50 nm. In the future itmay be possible to practice projection lithography down to about 5 nm.Also, by going to x-ray proximity lithography, it should be possible touse the techniques described herein for light sources down to about 0.5nm.

For projection lithography an active material would need to be chosenwhich would have at least one good emission line within the reflectivityrange of the mirrors used for the projection good lines are availablethroughout extreme UV spectrum. Good lines are also available in rangeswhich could apply down to 0.5 nm for the proximity lithography.Therefore, Applicants believe many or most of the concepts and ideasexpressed herein would be useful throughout the spectral range fromabout 0.5 nm to about 50 nm.

It is understood that the above described embodiments are illustrativeof only a few of the many possible specific embodiments which canrepresent applications of the principals of the present invention. Forexample, instead of recirculating the working gas it may be preferableto merely trap the lithium and discharge the helium. Use of otherelectrode-coating combinations other than tungsten and silver are alsopossible. For example copper or platinum electrodes and coatings wouldbe workable. Other techniques for generating the plasma pinch can besubstituted for the specific embodiment described. Some of these othertechniques are described in the patents referenced in the backgroundsection of this specification, and those descriptions are allincorporated by reference herein. Many methods of generating highfrequency high voltage electrical pulses are available and can beutilized. An alternative would be to keep the lightpipe at roomtemperature and thus freeze out both the lithium and the tungsten as itattempts to travel down the length of the lightpipe. This freeze-outconcept would further reduce the amount of debris which reached theoptical components used in the lithography tool since the atoms would bepermanently attached to the lightpipe walls upon impact. Deposition ofelectrode material onto the lithography tool optics can be prevented bydesigning the collector optic to re-image the radiation spot through asmall orifice in the primary discharge chamber and use a differentialpumping arrangement. Helium or argon can be supplied from the secondchamber through the orifice into the first chamber. This scheme has beenshown to be effective in preventing material deposition on the outputwindows of copper vapor lasers. Lithium hydride may be used in the placeof lithium. The unit may also be operated as a static-fill systemwithout the working gas flowing through the electrodes. Of course, avery wide range of repetition rates are possible from single pulses toabout 5 pulses per second to several hundred or thousands of pulses persecond. If desired, the adjustment mechanism for adjusting the positionof the solid lithium could be modified so that the position of the tipof the central electrode is also adjustable to account for erosion ofthe tip.

Many other electrode arrangements are possible other than the onesdescribed above. For example, the outside electrode could be cone shapedrather than cylindrical as shown with the larger diameter toward thepinch. Also, performance in some embodiments could be improved byallowing the inside electrode to protrude beyond the end of the outsideelectrode. This could be done with spark plugs or other preionizers wellknown in the art. Another preferred alternative is to utilize for theouter electrode an array of rods arranged to form a generallycylindrical or conical shape. This approach helps maintain a symmetricalpinch centered along the electrode axis because of the resultinginductive ballasting.

Accordingly, the reader is requested to determine the scope of theinvention by the appended claims and their legal equivalents, and not bythe examples which have been given.

1. A production line compatible, high repetition rate, high averagepower pulsed high energy photon source comprising: A. a pulse powersystem comprising a pulse transformer for producing electrical pulseswith duration in the range of 10 ns to 200 ns, B. a vacuum chamber, C.an active material contained in said vacuum chamber said active materialcomprising an atomic species characterized by an emission line within adesired extreme ultraviolet wavelength range, D. a hot plasma productionmeans for producing a hot plasma at a hot plasma spot in said vacuumchamber so as to produce at least a selected output wattage, averagedover at least the extreme ultraviolet radiation at wavelengths withinsaid desired extreme ultraviolet wavelength range, E. a radiationcollection and focusing means for collecting a portion of saidultraviolet radiation and focusing said radiation at a location distantfrom said hot plasma spot.
 2. A source as in claim 1 wherein said hotplasma production means is a dense plasma focus device.
 3. The source asin claim 2 wherein said dense plasma focus device comprises coaxialelectrodes.
 4. The source as in claim 3 and further comprising a gasinjection means for injecting active gas from a nozzle positioned on anopposite side of said hot plasma spot from said electrodes.
 5. Thesource as in claim 2 and further comprising a capacitor means chosen toproduce peak capacitor current during a plasma pinch event.
 6. Thesource as in claim 2 wherein said dense plasma focus device comprisescoaxial electrodes defining a central electrode.
 7. The source as inclaim 6 wherein said central electrode is an anode.
 8. The source as inclaim 7 wherein a portion of said anode is hollow and said anode definesa hollow tip dimension at a tip of said anode and said hollow portionbelow said tip is larger than said hollow tip dimension.
 9. The sourceas in claim 7 and further comprising a sacrifice region between saidelectrode to encourage post pinch discharge in a region away from a tipof said anode.
 10. The source as in claim 6 wherein said centralelectrode is water cooled.
 11. The source as in claim 6 and furthercomprising a heat pipe for cooling said central electrode.
 12. Thesource as in claim 6 wherein said electrodes are designed for radial rundown.
 13. A source as in claim 6 and further comprising a sputter sourcefor producing sputter material to replace material eroded from at leastone of said electrodes.
 14. A source as in claim 13 wherein said sputtersource also functions to provide preionization.
 15. The source us inclaim 6 wherein said central electrode is an anode defining outsidewalls and further comprising insulator material completely coveringanode walls facing said cathode.
 16. The source as in claim 15 whereinsaid anode also defines inner walls and comprising insulator materialcovering at least a portion of said inner walls.
 17. The source as inclaim 6 wherein said electrodes are comprised at least in part ofpyrolytic graphite.
 18. A source as in claim 2 and further comprising amagnetic means for applying a magnetic field to control at least onepinch parameters.
 19. A source as in claim 18 wherein said parameter ispinch length.
 20. A source as in claim 18 wherein said parameter ispinch shape.
 21. A source as in claim 18 wherein said parameter is pinchposition.
 22. A source as in claim 1 wherein said hot plasma productionmeans is a z-pinch device.
 23. A source as in claim 1 wherein said hotplasma production means is a hollow cathode z-pinch.
 24. A source as inclaim 1 wherein said hot plasma production means is a capillarydischarge device.
 25. A source as in claim 1 wherein said hot plasmaproduction means comprises an excimer laser providing a high repetitionrate short pulse laser beam for generating said plasma in said vacuumchamber.
 26. A source us in claim 1 wherein said hot plasma productionmeans comprises a plasma pinch device and an excimer laser producingpulsed ultraviolet laser beams directed at a plasma produced in part bysaid plasma pinch device.
 27. A source as in claim 1 wherein saidradiation collector comprises a parabolic collector.
 28. A source as inclaim 1 wherein said radiation collector comprises an ellipsoidalcollector.
 29. A source as in claim 1 wherein said radiation collectorcomprises a tandem ellipsoidal mirror system.
 30. A source as in claim 1wherein said radiation collector comprises a hybrid collector comprisingat least one ellipsoidal reflector unit and at least one hyperbolicreflector unit.
 31. A source as in claim 30 wherein said hybridcollector comprises at least two ellipsoidal reflector units and atleast two hyperbolic collector units.
 32. A source as in claim 31wherein said hybrid collector also comprises a multi-layer mirror unit.33. A source as in claim 32 wherein said multi-layer mirror unit is atleast partially parabolic.
 34. A source as in claim 1 and alsocomprising a debris shield having narrow passages aligned with said hotplasma spot for passage of EUV light and restricting passage of debris.35. A source as in claim 34 wherein said debris shield is comprised ofhardened material surrounding passage ways left by removal of skinnypyramid shaped forms.
 36. A source as in claim 34 wherein said debrisshield is comprised of welded hollow cones comprised of metal foil. 37.A source as in claim 34 wherein said debris shield is comprised of aplurality of thin laminated sheets slacked to create said passageways.38. The source as in claim 34 and also comprising a magnet for producinga magnetic field directed perpendicular to an axis of EUV beams forforcing charged particles into a curved trajectory.
 39. The source as inclaim 38 wherein said magnet is a permanent magnet.
 40. The source as inclaim 38 wherein said magnet is an electromagnet.
 41. The source as inclaim 34 wherein said debris shield is a honeycomb debris shield. 42.The source as in claim 41 wherein said honeycomb debris shield compriseshardened plasticized powder batch material.
 43. The source as in claim42 wherein said powder batch material is hardened by sintering.
 44. Thesource as in claim 34 and further comprising a gas control system tocreate a gas flow in said vacuum chamber through at least a portion ofsaid debris shield in a direction opposite a direction of EUV lightthrough said debris shield.
 45. The source as in claim 44 wherein gasflows through said debris shield in two directions.
 46. The source as inclaim 34 and further comprising a shutter with a seal located betweensaid debris shield and said radiation collector to permit replacement ofelectrodes and the debris shield without loss of vacuum around saidradiation collector.
 47. A source as in claim 46 and further comprisingan electrode set arranged as a module with said debris shield so thatthe electrode set and the debris shield can be easily replaced as aunit.
 48. The source as in claim 1 wherein said active material ischosen from a group consisting of xenon, tin, lithium, indium, cadmiumand silver.
 49. The source as in claim 1 wherein said vacuum chambercontains, in addition to said active material, a buffer gas.
 50. Thesource as in claim 49 wherein said buffer gas is chosen from a groupconsisting of helium and neon.
 51. The source as in claim 49 whereinsaid buffer gas comprises hydrogen.
 52. The source as in claim 1 whereinsaid active material is injected into said vacuum chamber through anelectrode.
 53. The source as in claim 1 wherein said active material isintroduced into said vacuum chamber as a compound.
 54. The source as inclaim 53 wherein the compound is chosen from a group consisting of Li2O,LiH, LiOH, LiCl, Li2CO3, LiF, CH3OLi and solutions of any materials inthis group.
 55. The source as in claim 1 and further comprising a laserfor vaporizing said active material.
 56. The source as in claim 1 andfurther comprising an RF source for sputtering active material into alocation within or near said hot plasma spot.
 57. The source as in claim1 and further comprising a preionization means.
 58. The source as inclaim 57 wherein said preionization means comprises spark plug typepins.
 59. The source as in claim 57 wherein said preionization meanscomprises an RF source.
 60. The source as in claim 57 wherein saidactive material is preionized prior to injection into said vacuumchamber.
 61. The source as in claim 60 wherein said preionization meanscomprises a radiation means for directing radiation to a nozzle topreionize active material prior to its leaving said nozzle to enter saidvacuum chamber.
 62. The source as in claim 1 wherein said activematerial is lithium contained in porous tungsten.
 63. The source as inclaim 62 and further comprising an RF means driving lithium atoms out ofsaid porous tungsten.
 64. The source as in claim 1 wherein said sourceis positioned to provide EUV light to a lithography machine.
 65. Thesource as in claim 64 wherein a portion of said source is integratedinto said lithography machine.
 66. A source as in claim 1 wherein saidmeans for producing a hot plasma is sufficient to produce at least 45.4Watts at an intermediate focus.
 67. A source as in claim 1 wherein saidmeans for producing a hot plasma is sufficient to produce at least 105.8Watts at an intermediate focus.
 68. A source as in claim 1 wherein saidactive material is chosen to produce EUV radiation within a wavelengthband within about 2% of 13.5 nm.
 69. A source as in claim 1 wherein saidpulse power system is operating at repetition rates of at least 6,000pulses per second.
 70. A source as in claim 1 wherein said pulse powersystem is operating at repetition rates of at least 10,000 pulses persecond.
 71. A source as in claim 1 wherein said radiation collector isdesigned to produce homogenization of said EUV radiation.
 72. A sourceas in claim 1 wherein said active material is delivered to regions ofsaid hot plasma spot as a metal in fluid form.
 73. A source as in claim72 wherein said fluid form is liquid.
 74. A source as in claim 72wherein said fluid form is a solution.
 75. A source as in claim 72wherein said fluid form is a suspension.
 76. A source as in claim 1wherein EUV light produced by electrons impact an electron material iscollected along with EUV light from said plasma hot spot.
 77. A sourceas in claim 1 wherein said active material is a metal vapor produced bysputtering.
 78. A source as in claim 1 wherein said active material ischosen to produce high energy radiation light in the range of 0.5 nm to50 nm.